Support 512K "RCT6" in BTT SKR Mini (#15890)

.travis.yml

Marlin/src/HAL/HAL_STM32F1/HAL.h

Marlin/src/pins/pins.h

Marlin/src/pins/stm32/pins_BIGTREE_SKR_E3_DIP.h

Marlin/src/pins/stm32/pins_BIGTREE_SKR_MINI_E3.h

Marlin/src/pins/stm32/pins_BIGTREE_SKR_MINI_V1_1.h

Marlin/src/pins/stm32/pins_BTT_SKR_MINI_E3_V1_2.h

...t/share/PlatformIO/ldscripts/STM32F103RC_SKR_MINI_256K.ld

...t/share/PlatformIO/ldscripts/STM32F103RC_SKR_MINI_512K.ld

buildroot/share/PlatformIO/scripts/STM32F103RC_SKR_MINI.py

buildroot/share/atom/auto_build.py

buildroot/share/tests/STM32F103RC_bigtree_NOUSB-tests

platformio.ini