arm: dts: mt7623: add support for Bananapi R2 (BPI-R2) board

Add support for the Bananapi R2 (BPI-R2) development board from
BIPAI KEJI. Detailed hardware information for BPI-R2 which could be
found on http://www.banana-pi.org/r2.html

The patch added nodes into the SoC-level file mt7623.dtsi such as CPU OPP
table and thermal zone treating CPU as one of cooling devices and also
added nodes into board-level file mt7623n-bananapi-bpi-r2.dts such as
MediaTek GMAC, MT7530 Switch, the crypto engine, USB, IR, I2S, I2C, UART,
SPI, PWM, GPIO keys, GPIO LEDs and PMIC LEDs. As to the other missing
hardware and peripherals, they would be added and integrated continuously.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>