soc: mediatek: cmdq: add assign function

Add assign function in cmdq helper which assign constant value into
internal register by index.

Signed-off-by: Dennis YC Hsieh <dennis-yc.hsieh@mediatek.com>
Link: https://lore.kernel.org/r/1592749115-24158-3-git-send-email-dennis-yc.hsieh@mediatek.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>