cgtqmx6: disable FEC when onboard ethernet is disabled

FEC is the on-chip "Freescale Ethernet Controller" in i.MX6 SOC and has
a dependency for the ethernet MII interface. If onboard Ethernet is
disabled, FEC have to be disabled too otherwise uboot won't build.

Signed-off-by: Georg Hartinger <georg.hartinger@congatec.com>