ARM: dts: msm: Add USB VBUS supply to MSM8992

The USB controller can operate in host mode, in which case
it uses 5V VBUS supply from the PMI8994 SMB charger regulator.
Add the regulator node to the USB device tree entry.

Change-Id: I914d8b6a500d9da3825da9714117b4ad38e966fc
Signed-off-by: Jack Pham <jackp@codeaurora.org>