arm64: dts: mt8173-evb: Add PMIC support

The MT8173 eval board contains a MT6397 PMIC. This adds the
corresponding device node to the dts file.

Signed-off-by: Henry Chen <henryc.chen@mediatek.com>
Signed-off-by: Eddie Huang <eddie.huang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>