soc: mediatek: SCPSYS: Add regulator support

The power domains are supplied by regulators. Add support for them so
that the regulators are properly turned on before a domain is powered up
and turned off when a domain is powered down.

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