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

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