cpu/stm32: add STM32U3 peripheral driver support

[itsadarshnair]

Contribution description

Follow-up to the STM32U3 / NUCLEO-U385RG-Q bring-up (#), stacked on
that PR. It adds the remaining MCU peripherals on top of the bring-up base.

Peripherals added (drivers in cpu/stm32, board wiring in
boards/nucleo-u385rg-q):

• SPI — SPI1 on PA7/PA6/PA5; includes a fix for last-frame RX truncation on U3
  (continuous mode + CSUSP). Verified on hardware via loopback test.
• PWM — TIM2 (CH1 User LED / PA5, CH2 PB3, CH3 PB10, CH4 PB11).
• RTC — register/EXTI mappings for U3; uses the on-board 32.768 kHz LSE.
• flashpage — page size / write block / FLASH key for U3; VCORE voltage range 1
  is selected so program/erase does not raise PGSERR.
• VBAT — internal VBAT channel, with the U3 scaling/reference fixed.
• HWRNG — U3 family whitelisted/routed.
• ADC — adc_f3_h7.c renamed to adc_f3_h7_u3.c and extended with the U3 path;
  ADC1 on Arduino A0–A5 plus internal VREFINT.
• I2C — I2C1 on PB8/PB9, APB1 routing and register fixups.
• USB device — USB DRD FS support in usbdev_fs (D+/D− on PA12/PA11).

Warning

ADC, I2C, and USB are work-in-progress: they are configured and compile, but
still have known bugs and are not yet verified on hardware. They are included here
so the board configuration is complete and to allow collaborative debugging, but
should be treated as experimental until fixed and tested.

Testing procedure

Note

Depends on # — review/merge that first. Flashing uses
STM32CubeProgrammer (OpenOCD does not yet support the STM32U3); see the bring-up
PR / board doc.md for the exact command.

Compiled for the NUCLEO-U385RG-Q:

cd tests/periph/spi && make BOARD=nucleo-u385rg-q
cd tests/periph/pwm && make BOARD=nucleo-u385rg-q
cd tests/periph/rtc && make BOARD=nucleo-u385rg-q
cd tests/periph/flashpage && make BOARD=nucleo-u385rg-q
cd tests/periph/adc && make BOARD=nucleo-u385rg-q
cd tests/periph/i2c && make BOARD=nucleo-u385rg-q
cd examples/advanced/usbus_minimal && make BOARD=nucleo-u385rg-q

Per-peripheral status:

Peripheral	Status	Notes
SPI	working	SPI1; verified via loopback test
PWM	working	TIM2 channels
RTC	working	on-board LSE
flashpage	working	program/erase + verify (VCORE range 1)
VBAT	working	internal VBAT channel, scaling fixed
ADC	buggy	external channels A0–A5 not reading correctly; under investigation
I2C	buggy	configured/compiles; bus scan can hang; under inve
USB device	buggy	configured/compiles; not yet working; under investigation

How to confirm it's not in master: these peripherals are not provided by the board
on master (the board and stm32u3 family don't exist there); after this P
make BOARD=nucleo-u385rg-q builds the apps above. The "working" peripherals behave
as their tests expect; ADC/I2C/USB still need fixes (see below).

Issues/PRs references

Depends on #22427 (initial bring-up). Together these two PRs supersede
the original #22175.Do not merge until #22427 is merged

Declaration of AI-Tools / LLMs usage:

AI-Tools / LLMs that were used are:

• Claude Code with Claude Opus 4.8 for code generation, with user review
• Gemini for documentation generation, with user review

[github-actions]

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[riot-ci]

Murdock results

✔️ PASSED

c8a1cad boards/nucleo-u385rg-q: enable peripherals

Success	Failures	Total	Runtime
11124	0	11124	12m:16s

Artifacts

• Documentation preview