arm: dts: mt7623: extend common file reused by all boards with MT7623 SoCs

Move all possible setups for pio into SoC level DTSI file mt7623.dtsi in
order to introduce more boards such as official MT7623A reference boards
without copy-n-pasting almost the same content of nodes in pio into every
new file.

So, it should be better to reuse those nodes by consolidating them into
the common file mt7623.dtsi from the current existent DTS and allow new
DTS files to refer to them.

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