Use modified SoftwareSerialM which works with SKR Pro (#15796)

platformio.ini

 extra_scripts = pre:buildroot/share/PlatformIO/scripts/generic_create_variant.py
 build_flags   = ${common.build_flags}
   -DUSBCON -DUSBD_USE_CDC -DUSBD_VID=0x0483 -DUSB_PRODUCT=\"STM32F407ZG\"
-  -DTARGET_STM32F4 -DSTM32F407_5ZX -DVECT_TAB_OFFSET=0x8000 -DHAVE_HWSERIAL6
+  -DTARGET_STM32F4 -DSTM32F407_5ZX -DVECT_TAB_OFFSET=0x8000 -DHAVE_HWSERIAL6 -DSS_TIMER=4
 lib_deps      =
   U8glib-HAL=https://github.com/MarlinFirmware/U8glib-HAL/archive/bugfix.zip
   LiquidCrystal@1.3.4
-  TMCStepper@>=0.5.0,<1.0.0
+  TMCStepper@>=0.5.2,<1.0.0
   Adafruit NeoPixel
   LiquidTWI2=https://github.com/lincomatic/LiquidTWI2/archive/master.zip
   Arduino-L6470=https://github.com/ameyer/Arduino-L6470/archive/dev.zip
+  SoftwareSerialM=https://github.com/sjasonsmith/SoftwareSerialM/archive/SKR_PRO.zip
 src_filter    = ${common.default_src_filter} +<src/HAL/HAL_STM32>
 monitor_speed = 250000