drivers: mbox: stm32-hsem: build driver if DT enabled

Default to building the driver if its node is enabled in
the device-tree, avoiding the need to manually set
CONFIG_MBOX_STM32_HSEM.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>

Author: Alain Volmat

drivers/mbox/Kconfig.stm32_hsem

@@ -3,6 +3,7 @@
 
 config MBOX_STM32_HSEM
 	bool "MBOX STM32 HSEM driver"
+	default y
 	depends on DT_HAS_ST_MBOX_STM32_HSEM_ENABLED
 	help
 	  MBOX Driver for STM32 hardware semaphore