rockchip: dts: rock64: Fix XHCI usage

If the VBUS regulator is always-on, XHCI will fail to detect USB 3.0
devices; USB 2.0 devices will work however.

Make the VBUS regulator controllable and tie it to only the XHCI. This
makes all three USB ports usable.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>