arm: dts: mt2701: Add auxadc device node.

Add auxadc device node for MT2701.

Signed-off-by: Zhiyong Tao <zhiyong.tao@mediatek.com>
Signed-off-by: Erin Lo <erin.lo@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>