cpu/stm32: add initial STM32U3 and nucleo-u385rg-q support

[itsadarshnair]

Contribution description

This PR adds initial support for the STM32U3 MCU family (Cortex-M33, TrustZone)
and the NUCLEO-U385RG-Q board, covering the minimum needed to boot RIOT and
interact with it over a serial console.

Parts of RIOT involved:

• cpu/stm32: new u3 CPU family — CPU_FAM/line/RAM/ROM detection
  (stm32_info.mk, stm32_mem_lengths.mk), CMSIS header selection, Kconfig
  family/model definitions, and the cpu_stm32u3 feature.
• Clock: stmclk_u3.c + clk/u3/cfg_clock_default.h. The U3 reaches high SYSCLK
  via the MSI oscillator (no PLL bring-up needed); FLASH wait states are wired to
  the ACR register to avoid HardFaults at higher frequencies.
• Core register fixups so the family compiles: APB3 / AHB1ENR1-2 enable routing,
  FLASH key, EXTI/SYSCFG, and CLOCK_LSI (periph/cpu_common.h, periph_cpu.h,
  periph/u3/periph_cpu.h).
• Peripherals in this PR: GPIO (incl. EXTI external interrupts) and
  UART/LPUART (USART1 = ST-LINK VCP / STDIO, LPUART1), plus the general-purpose
  timer backend.
• boards/nucleo-u385rg-q: new thin Nucleo-64 board (LED/button, clock selection,
  serial console pins).

Additional peripherals (ADC, SPI, I2C, PWM, USB, RTC, VBAT, HWRNG, flashpage) are
added in a follow-up PR stacked on top of this one.

Testing procedure

These apps were compiled for the NUCLEO-U385RG-Q and run on hardware:

cd examples/basic/blinky && make BOARD=nucleo-u385rg-q
cd examples/basic/default && make BOARD=nucleo-u385rg-q
cd tests/periph/uart && make BOARD=nucleo-u385rg-q
cd tests/periph/timer_periodic && make BOARD=nucleo-u385rg-q
cd tests/periph/gpio_ll && make BOARD=nucleo-u385rg-q

Note

OpenOCD does not yet support the STM32U3, so make flash (default programmer:
openocd) does not work for this board. Until OpenOCD support lands, flash the
built ELF with the STM32CubeProgrammer CLI, e.g.:

STM32_Programmer_CLI -c port=SWD mode=Normal reset=HWrst freq=8000 \
    -e all \
    -w bin/nucleo-u385rg-q/<application>.elf \
    -v -rst

After flashing, open the serial console (ST-LINK VCP, 115200 baud) with
make BOARD=nucleo-u385rg-q term.

Expected results:

• blinky — the green User LED (LD2) blinks, confirming clock + GPIO bring-up.
• default — the RIOT shell comes up on the VCP and responds to commands,
  confirming the UART/STDIO console.
• tests/periph/uart — UART/LPUART behaves as the test expects.
• tests/periph/timer_periodic — the periodic timer fires at the expected intervals.
• tests/periph/gpio_ll — GPIO including external interrupts (EXTI) works.

None of this is available in master today — the stm32u3 CPU family and the
nucleo-u385rg-q board do not exist there, so make BOARD=nucleo-u385rg-q fails
on master with an unknown board.

Issues/PRs references

Supersedes #22175 (that PR is being split into this focused bring-up PR plus a
peripherals PR for easier review). The peripherals follow-up is stacked on this
branch and depends on it.

Declaration of AI-Tools / LLMs usage:

AI-Tools / LLMs that were used are:
-Claude for code generation but reviewed by user.
-Gemini for planning and review.

[github-actions]

Hey @itsadarshnair, thank you for your first contribution! We really appreciate it! If you haven't already, please take a look at our contributing guidelines before the review process starts. Also, due to how the GitHub review system works, please avoid force-pushing or squashing your commits unless asked to by a maintainer (or unless your commit is still in "draft commit" stage). Lastly, make sure to comply with our AI Policy when using AI. Your pull request will be reviewed as soon as possible.

[riot-ci]

Murdock results

✔️ PASSED

2b65641 boards/nucleo-u385rg-q: document STM32CubeProgrammer flashing

Success	Failures	Total	Runtime
11123	0	11124	11m:39s

Artifacts

• Documentation preview