arm64: dts: mt8173: Fixup pinctrl nodes

The 8173 pinctrl node doesn't follow dts convention. Fix them.
Also add a comment to explain pinctrl register usage to make it
more clear.

Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>