staging: mt7621-eth: Document ralink/mediatek SoC ethernet binding

Add possible dt binding for mediatek gigabit switches.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Michael Lee <igvtee@gmail.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>