net: fec_mxc: add 1000 Mbps selection

Define FEC_QUIRK_ENET_MAC and add to
arch-mx6/imx-regs.h

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>