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>

Documentation/devicetree/bindings/soc/mediatek/scpsys.txt

@@ -21,6 +21,18 @@ Required properties:
 		      These are the clocks which hardware needs to be enabled
 		      before enabling certain power domains.
 
+Optional properties:
+- vdec-supply: Power supply for the vdec power domain
+- venc-supply: Power supply for the venc power domain
+- isp-supply: Power supply for the isp power domain
+- mm-supply: Power supply for the mm power domain
+- venc_lt-supply: Power supply for the venc_lt power domain
+- audio-supply: Power supply for the audio power domain
+- usb-supply: Power supply for the usb power domain
+- mfg_async-supply: Power supply for the mfg_async power domain
+- mfg_2d-supply: Power supply for the mfg_2d power domain
+- mfg-supply: Power supply for the mfg power domain
+
 Example:
 
 	scpsys: scpsys@10006000 {