arm64: dts: librem5-devkit: add a vbus supply to usb0

Without a VBUS supply the dwc3 driver won't go into otg mode.

Fixes: eb4ea085 ("arm64: dts: fsl: librem5: Add a device tree for the Librem5 devkit")
Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>