arm: dts: MediaTek: add device tree for MT7623

This adds device tree for MT7623 development board - Bananapi R2
Detailed hardware information for BPI-R2 which could be found on
http://wiki.banana-pi.org/Banana_Pi_BPI-R2.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Tested-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>