ARM: dts: msm: Add USB VBUS regulator for MSM8994

PMI8994 supports supplying 5V USB VBUS for host mode. Add
regulator-name node in order to enable it. Add a reference
to this regulator in the USB device.

Change-Id: Id104abc807a23f673c74823dd1de45e0c18cd840
Signed-off-by: Jack Pham <jackp@codeaurora.org>