arm: dts: mt7623: add MT7623A SoC level DTS

Add a common file for MT7623A SoC level DTS, indicating MT7623A only
has a specific definition for power domain. That causes we need to change
related consumers devices such as audio, ethernet, crypto, NAND, and USB
controller to grasp its own power domain it should belong to.

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