mips: mtmips: add two reference boards for mt7620

The mt7620_rfb board supports integrated 10/100M PHYs plus two external
giga PHYs. It also has 8MB SPI-NOR, mini PCI-e x1 slot, SDHC and USB.

The mt7620_mt7530_rfb boards supports an external MT7530 giga switch and a
16MB SPI-NOR flash.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>