net: fec: do not access reserved register for i.MX8M

The MIB RAM and FIFO receive start register does not exist on
i.MX8M. Accessing these register will cause system hang.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>