net: Kconfig: FEC: Add dependency on i.MX8M

Make FEC driver could be used by i.MX8M when CONFIG_FEC_MXC defined
in defconfig.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>