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

Add clocks needed by Mediatek VENC and VENC_LT power domianis.
These clocks were needed by accessing subsystem's registers,
so they need to be enabled before power on these subsystems.

Signed-off-by: James Liao <jamesjj.liao@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>