stm32: uart: Support for wakeup from stop

[ycsin]

The STM32's UART driver currently lacks the ability to wake up from stop mode. For low power application that requires to listen for UART RX (modem, sensors using uart), the applications needs to delicately constraint the PM mode while the MCU is communicating with these external peripherals and unconstraint after the transaction is finished. This makes the LPM not as easy to use, and for applications where the MCU should be in LP mode unless the RX receives something, this means that LPM cant be enabled at all.

Describe the solution you'd like
Add support for wake up from stop mode for the STM32's UART driver.

Describe alternatives you've considered
I've tried to implement this feature but stumbled upon the step where I need to switch the clock source of WKUP capable UART instances to HSI16 using LL_RCC_SetUSARTClockSource before the MCU goes to sleep, this function expects predefined macros which can be different across MCUs.

Reference(s)

• https://github.com/STMicroelectronics/STM32CubeG0/blob/master/Projects/NUCLEO-G0B1RE/Examples_LL/LPUART/LPUART_WakeUpFromStop_Init/Src/main.c

[erwango]

I'm currently working on #42097 which should allow to configure alternate clocks like HSI16. At least in a first step, this would not be dynamic, but would allow to configure the right clock that enables to enter STOP mode.
There are still many steps from #42097 to what you exactly need, but that's the way to go and work is started.

[erwango]

Fixed by #48640