ARM: dts: rockchip: Setup usb vbus-supply on rk3288-rock2

Now that the rockchip usb phy has a vbus-supply property use that to
control the vbus regulator on rock2.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>