net: fec: add necessary defines to work on ARM64

The i.MX8 is a ARMv8 based SoC, that uses the same FEC IP as the
earlier, ARMv7 based, i.MX SoCs. Allow the driver to work on ARM64.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>