drivers/soft_i2c: add soft_i2c_as_periph_i2c

With the new feature it should be possible to just use a software
I2C bus instead of a `periph_i2c` using the same APIs.

I tested this using the following patch using an SHT3X breakout board
connected to an nRF52840-DK:

```diff
--- a/tests/drivers/sht3x/Makefile
+++ b/tests/drivers/sht3x/Makefile
@@ -2,4 +2,19 @@ include ../Makefile.drivers_common

 USEMODULE += sht3x

+PIN_SCL ?= GPIO_PIN(0,27)
+PIN_SDA ?= GPIO_PIN(0,26)
+
+ifneq (,$(PIN_SCL))
+  CFLAGS += "-DSOFT_I2C_PARAM_SCL=$(PIN_SCL)"
+endif
+ifneq (,$(PIN_SDA))
+  CFLAGS += "-DSOFT_I2C_PARAM_SDA=$(PIN_SDA)"
+endif
+
+USEMODULE += soft_i2c
+USEMODULE += soft_i2c_as_periph_i2c
+
+CFLAGS += "-DSHT3X_PARAM_I2C_ADDR=SHT3X_I2C_ADDR_1"
+
 include $(RIOTBASE)/Makefile.include
```

drivers/soft_i2c/soft_i2c.c

@@ -837,3 +837,55 @@ void auto_init_soft_i2c(void)
         soft_i2c_init(SOFT_I2C_DEV(i));
     }
 }
+
+/* provide periph_i2c API via soft_i2c as weak symbols */
+__attribute__((weak, alias("soft_i2c_init")))
+void i2c_init(i2c_t dev);
+
+__attribute__((weak, alias("soft_i2c_init_pins")))
+void i2c_init_pins(i2c_t dev);
+
+__attribute__((weak, alias("soft_i2c_deinit_pins")))
+void i2c_deinit_pins(i2c_t dev);
+
+__attribute__((weak, alias("soft_i2c_pin_sda")))
+gpio_t i2c_pin_sda(i2c_t dev);
+
+__attribute__((weak, alias("soft_i2c_pin_scl")))
+gpio_t i2c_pin_scl(i2c_t dev);
+
+__attribute__((weak, alias("soft_i2c_acquire")))
+void i2c_acquire(i2c_t dev);
+
+__attribute__((weak, alias("soft_i2c_release")))
+void i2c_release(i2c_t dev);
+
+__attribute__((weak, alias("soft_i2c_read_reg")))
+int i2c_read_reg(i2c_t dev, uint16_t addr, uint16_t reg,
+                 void *data, uint8_t flags);
+
+__attribute__((weak, alias("soft_i2c_read_regs")))
+int i2c_read_regs(i2c_t dev, uint16_t addr, uint16_t reg,
+                  void *data, size_t len, uint8_t flags);
+
+__attribute__((weak, alias("soft_i2c_read_byte")))
+int i2c_read_byte(i2c_t dev, uint16_t addr, void *data, uint8_t flags);
+
+__attribute__((weak, alias("soft_i2c_read_bytes")))
+int i2c_read_bytes(i2c_t dev, uint16_t addr,
+                   void *data, size_t len, uint8_t flags);
+
+__attribute__((weak, alias("soft_i2c_write_byte")))
+int i2c_write_byte(i2c_t dev, uint16_t addr, uint8_t data, uint8_t flags);
+
+__attribute__((weak, alias("soft_i2c_write_bytes")))
+int i2c_write_bytes(i2c_t dev, uint16_t addr, const void *data,
+                    size_t len, uint8_t flags);
+
+__attribute__((weak, alias("soft_i2c_write_reg")))
+int i2c_write_reg(i2c_t dev, uint16_t addr, uint16_t reg,
+                  uint8_t data, uint8_t flags);
+
+__attribute__((weak, alias("soft_i2c_write_regs")))
+int i2c_write_regs(i2c_t dev, uint16_t addr, uint16_t reg,
+                  const void *data, size_t len, uint8_t flags);

makefiles/features_check.inc.mk

@@ -1,5 +1,9 @@
 # Check if all required FEATURES are provided
 
+ifneq (,$(filter soft_i2c,$(USEMODULE)))
+  FEATURES_PROVIDED += periph_i2c
+endif
+
 # Features that are used without taking "one out of" dependencies into account
 FEATURES_USED_SO_FAR := $(sort $(FEATURES_REQUIRED) $(FEATURES_OPTIONAL_USED))
 

makefiles/features_modules.inc.mk

@@ -54,6 +54,11 @@ PERIPH_IGNORE_MODULES := \
   periph_wdog \
   periph_wdt_auto_start \
   #
+
+ifneq (,$(filter soft_i2c_as_periph_i2c,$(USEMODULE)))
+  USEMODULE := $(filter-out periph_i2c,$(USEMODULE))
+endif
+
 PERIPH_MODULES := $(filter-out $(PERIPH_IGNORE_MODULES),\
                                $(filter periph_%,$(USEMODULE)))
 # Use simple expansion to avoid USEMODULE referencing itself

makefiles/pseudomodules.inc.mk

@@ -534,6 +534,11 @@ PSEUDOMODULES += sock_ip
 PSEUDOMODULES += sock_tcp
 PSEUDOMODULES += sock_udp
 PSEUDOMODULES += socket_zep_hello
+## @defgroup drivers_soft_i2c_as_periph_i2c Use `soft_i2c` to provide `periph_i2c`
+## @ingroup drivers_soft_i2c
+## @{
+PSEUDOMODULES += soft_i2c_as_periph_i2c
+## @}
 PSEUDOMODULES += soft_uart_modecfg
 PSEUDOMODULES += stdin
 PSEUDOMODULES += stdio_available