ARM: dts: sun8i: reference-design-tablet: Add drivevbus-supply

Add a drivevbus-supply property so that the drivevbus regulator
reports the right voltage value.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>