boards: nucleo_h755zi_q: add CAN support on M7

Add CAN support by configuring fdcan1 in the device tree for the M7
core. As with other H7xx boards, it will be clocked by pll2 which is
also turned on.

Updated documentation.

Tested on nucleo_h755zi_q board.

Signed-off-by: Martin Thiede <martin.thiede@accenture.com>