Fysetc S6 support (#15850)

Marlin/src/core/boards.h

Marlin/src/pins/pins.h

Marlin/src/pins/stm32/pins_FYSETC_S6.h

buildroot/share/PlatformIO/boards/fysetc_s6.json

buildroot/share/PlatformIO/scripts/fysetc_STM32S6.py

...root/share/PlatformIO/variants/FYSETC_S6/PeripheralPins.c

buildroot/share/PlatformIO/variants/FYSETC_S6/PinNamesVar.h

buildroot/share/PlatformIO/variants/FYSETC_S6/ldscript.ld

buildroot/share/PlatformIO/variants/FYSETC_S6/variant.cpp

buildroot/share/PlatformIO/variants/FYSETC_S6/variant.h

config/examples/FYSETC/S6/Configuration.h

config/examples/FYSETC/S6/Configuration_adv.h

platformio.ini