dt-binding: i2c: mt65xx: add vbus-supply property

Add vbus-supply property for mt65xx. The regulator can be passed into
core and turned off during suspend/sleep to reduce power consumption.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>