doc: mediatek: add documentation for mt7621 reference boards

The MT7621 requires external binary blob being executed during u-boot's
boot-up flow. It's necessary to provide a guide here for users to correctly
build the u-boot.

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