Documentation: DT: net: add docs for ralink/mediatek SoC ethernet binding

Add three files. ralink,rt2880-net.txt  descibes the actual frame engine
and the other two describe the switch forntend bindings.

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
Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux-mediatek@lists.infradead.org
Cc: John Crispin <blogic@openwrt.org>
Cc: Felix Fietkau <nbd@nbd.name>
Cc: Michael Lee <igvtee@gmail.com>
Cc: steven.liu@mediatek.com
Cc: Fred.Chang@mediatek.com
Patchwork: https://patchwork.linux-mips.org/patch/11970/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>