board: atmel: sama5d27_wlsom1_ek: Set ethaddr from spi-nor flash

The SST26VF064BEUI spi-nor flash is programmed at the factory with a
globally unique address stored in the SFDP vendor parameter table and
it is permanently writeprotected. Retrieve the EUI-48 address and set it
as ethaddr env.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>