mx6sabre_common: Remove FEC related settings

In preparation for converting to DM_ETH and moving the FEC symbols
to Kconfig we need to move the FEC definitions to mx6sabreauto.h
and mx6sabresd.h to avoid build breakage during the conversion.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Alifer Moraes <alifer.wsdm@gmail.com>