usb: host: xhci-rockchip: use fixed regulator to control vbus

Use fixed regulator to control the voltage of vbus. Enable vbus
supply when usb start and disable vbus supply when usb stop.

Signed-off-by: Meng Dongyang <daniel.meng@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>