mips: mtmips: add two reference boards for mt7621

The mt7621_rfb board supports integrated giga PHYs plus one external
giga PHYs. It also has up to 512MiB DDR3, 16MB SPI-NOR, 3 mini PCI-e x1
slots, SDXC and USB.

The mt7621_nand_rfb board is almost the same as mt7621_rfb board, but it
uses NAND flash and SDXC is not available.

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