ARM: imx8: Add missing FEC ENET quirk for i.MX8/i.MX8X

ARM: imx8: Add missing FEC ENET quirk for i.MX8/i.MX8X

Both NXP SoCs i.MX8 and i.MX8X have ENET gigabit MAC.
Define FEC_QUIRK_ENET_MAC for the imx8 platform and remove this
definition from configs of boards, based on MX8/MX8X.

Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Acked-by: Marek Vasut <marex@denx.de>
Reviewed-By: Ramon Fried <rfried.dev@gmail.com>
Acked-by: Oliver Graute <oliver.graute@kococonnector.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>