SoftwareSerialM for SKR STM32F1 boards (#15875)

Marlin/src/HAL/HAL_AVR/HAL.h

Marlin/src/HAL/HAL_ESP32/WebSocketSerial.h

Marlin/src/HAL/HAL_ESP32/i2s.cpp

Marlin/src/HAL/HAL_ESP32/ota.cpp

Marlin/src/HAL/HAL_ESP32/timers.cpp

Marlin/src/HAL/HAL_ESP32/timers.h

Marlin/src/HAL/HAL_LPC1768/HAL_SPI.cpp

Marlin/src/HAL/HAL_LPC1768/persistent_store_flash.cpp

...HAL/HAL_LPC1768/u8g/u8g_com_HAL_LPC1768_st7920_sw_spi.cpp

...in/src/HAL/HAL_LPC1768/u8g/u8g_com_HAL_LPC1768_sw_spi.cpp

Marlin/src/HAL/HAL_LPC1768/watchdog.cpp

Marlin/src/HAL/HAL_SAMD51/HAL.cpp

Marlin/src/HAL/HAL_STM32/HAL.cpp

Marlin/src/HAL/HAL_STM32/timers.cpp

Marlin/src/HAL/HAL_STM32F1/HAL.cpp

Marlin/src/HAL/HAL_STM32F1/SoftwareSerial.h

Marlin/src/HAL/HAL_STM32F1/inc/SanityCheck.h

Marlin/src/HAL/HAL_STM32_F4_F7/HAL.cpp

Marlin/src/core/drivers.h

.../PlatformIO/variants/BIGTREE_SKR_PRO_1v1/PeripheralPins.c