net: fec: add defer probe for of_get_mac_address

If MAC address read from nvmem efuse by calling .of_get_mac_address(),
but nvmem efuse is registered later than the driver, then it
return -EPROBE_DEFER value. So modify the driver to support
defer probe when read MAC address from nvmem efuse.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>