stm32/uart: Enable UART FIFO for STM32N6.

[iabdalkader]

Enable UART FIFO for STM32N6.

Summary

Enable UART FIFO for N6.

Testing

Tested with baudrate 921600.

[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32

[dpgeorge]

This looks OK, but it doesn't link. The hal_uart_ex.c file is needed.

[iabdalkader]

This looks OK, but it doesn't link. The hal_uart_ex.c file is needed.

Added. Perhaps you should add an N6 board to the CI.

[dpgeorge]

Perhaps you should add an N6 board to the CI.

Yes, there are a few MCU series that could be added to CI: G0, G4, L1, N6, WL.