rockchip: dts: rk3328: add fixed regulator node for xhci

The driver changes gpio to fixed regulator to control vbus, so add
fixed regulator node in DTS for xhci driver.

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>