arm: dts: mt7623: add MT7623A reference boards

Add MT7623A reference board with eMMC and NAND, respectively.

The both boards compared against MT7623N BPI-R2, we could see there are
UART[0-1] and USB2 being removed, I2C2 and SPI1 being added, I2C1, UART2
owning distinct pin usage and an extra WM8960 codec chip plugged into the
I2C1 offering the functionality of audio player and recorder through
SoC audio front-end engine (AFE).

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Suggested-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>