rockchip: dts: rk3399-puma: replace 'rockchip, vbus-gpio' with fixed regulator

On the RK3399-Q7, we need to turn on the on-module USB hub before using the
USB host interfaces (only the OTG interface is directly connected to the edge
connector).  This drops the deprecated 'rockchip,vbus-gpio' property and uses
a fixed regulator to turn on the USB hub.

References: 26a8b80f "usb: host: xhci-rockchip: use fixed regulator to control vbus"
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>