boards: arduino: portenta_h7: Add FMC and SDRAM1 nodes.

Enabled the flash memory controller node along with the necessary pin
muxing. This allows us to interface with the additional 8MB of external
SDRAM available on the Portenta H7 board.

This has been tested minimally with the tests/drivers/memc/ram test.

Signed-off-by: Rahul Arasikere <arasikere.rahul@gmail.com>