dt-bindings: ARM: Mediatek: document binding for the PMIC wrapper

This adds documentation for the PMIC wrapper unit found on Mediatek
SoCs. Currently support are the MT8135 and MT8173 SoCs, but the PMIC
wrapper can also be found on MT6xxx and possibly other SoCs.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>