soc: mediatek: mtk-svs: use common function to disable restore voltages

The timing of disabling SVS bank and restore default voltage is more
than one place. Therefore, add a common function to use for removing
the superfluous codes.

Signed-off-by: Roger Lu <roger.lu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230202124104.16504-3-roger.lu@mediatek.com


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