net: gem: Allow to set the MAC from an EEPROM

Provide board specific option how to read MAC address from ROM.
Do it in generic way to be reusable by differnet boards.
If this is not enough board specific functions can be created.

Signed-off-by: Joe Hershberger <joe.hershberger@gmail.com> # driver part
Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>