include: configs: *imx8*: remove IMX_FEC_BASE

The IMX_FEC_BASE value is not used when CONFIG_DM_ETH is configured. So this
value can be removed.

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-By: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>