ARM: imx: add FEC sleep mode callback function

i.MX6q/dl, i.MX6SX SOCs enet support sleep mode that magic packet can
wake up system in suspend status. For different SOCs, there have some
SOC specifical GPR register to set sleep on/off mode. So add these to
callback function for driver.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>