drivers: ethernet: sam_gmac: Convert to using dts for I2C EEPROM

Introduce a simple binding for atmel,24mac402 EEPROM that the SAM
GMAC ethernet driver can utilize to get MAC address out of.  We
introduce a 'mac-eeprom' phandle into GMAC ethernet devicetree
node that will provide a pointer to the MAC eeprom to utilize.

Signed-off-by: Kumar Gala <galak@kernel.org>