net: fec: iMX6 FEC does not support half-duplex gigabit

See http://lists.infradead.org/pipermail/linux-arm-kernel/2014-July/270174.h

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
fec: Add disable_giga parameter to force 10/100 operation

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>