Add STM32H7R/S support

[dpoettler]

Board description (please complete the following information):

• Name: NUCLEO-H7S3L8
• Link to all relevant information:
  • https://www.st.com/resource/en/user_manual/um3276-stm32h7rx7sx-nucleo144-board-mb1737-stmicroelectronics.pdf
  • https://www.st.com/resource/en/schematic_pack/mb1737-h7s3l8-b02-schematic.pdf
  • ST, DigiKey, Mouser

Tried adding it myself but couldn't follow the guide sadly

[fpistm]

Hi @dpoettler
Before adding the board the STM32H7R/S series have to be added as it has its own STM32Cube: https://github.com/STMicroelectronics/STM32CubeH7RS
Moreover, it will require extra effort to support as it would probably required to support a bootloader in the 64k bootflash.