boards: stm32h7s78_dk: add FDCAN1 support

Add FDCAN1 configuration to the board's .dtsi file and
comments explaining how to enable it, since fdcan1 and i2c1
share the pb9 pin. Updated documentation to explain CAN usage.

Signed-off-by: Benjamin Klaric <benjamin.klaric01@gmail.com>
2 files changed