net: ks8851: Implement EEPROM MAC address readout

In case there is an EEPROM attached to the KS8851 MAC and the EEPROM
contains a valid MAC address, the MAC address is loaded into the NIC
registers on power on. Read the MAC address out of the NIC registers
and provide it to U-Boot.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>