i2c: mediatek: mt65xx: add optional vbus-supply

Add vbus-supply which provides power to SCL/SDA. Pass this regulator
into core so it can be turned on/off for low power mode support.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Qii Wang <qii.wang@mediatek.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>