board: microchip: set mac address for ethernet1 on icicle

The dts sync from Linux leaves mac0/ethernet1 enabled on icicle, but
U-Boot does not currently set a mac address for it. Expand on the code
which currently sets the mac for mac1/ethernet0 to optionally set the
mac address for the second ethernet.

Reviewed-by: Padmarao Begari <padmarao.begari@microchip.com>
Tested-by: Padmarao Begari <padmarao.begari@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>