board: stm32: add stm32h747-discovery board support

board: stm32: add stm32h747-discovery board support

The board includes an STM32H747XI SoC with the following resources:
 - 2 Mbytes Flash
 - 1 Mbyte SRAM
 - LCD-TFT controller
 - MIPI-DSI interface
 - FD-CAN
 - USB 2.0 high-speed/full-speed
 - Ethernet MAC
 - camera interface

Detailed information can be found at:
https://www.st.com/en/evaluation-tools/stm32h747i-disco.html

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>