net: zynq_gem: Load ethaddr from zynq_gem registers if ethaddr is not defined in environment

For board that has MAC address stored in EEPROM, FSBL will load Mac Address into zynq_gem.
In U-Boot, if user does not specify a MAC address, U-Boot should load the Mac Address
from zynq_gem registers.

Signed-off-by: Tinghui Wang <steven.wang@digilentinc.com>
Xilinx: Arm: Add Config Support for ZYBO

This patch adds config support needed for ZYBO target and has
been tested on Rev. B ZYBO hardware.

Signed-off-by: Tinghui Wang <steven.wang@digilentinc.com>