dt-bindings: soc: Add supplies for Mediatek SCPSYS unit

The power domains in the SCPSYS unit are supplied by regulators. Add the
properties for them to the binding document.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>