arm/dt: msm9625: Add a stub regulator for USB VBUS 5V control

Add a stub fixed-regulator device for MSM9625 which is used by
the USB device to satisfy its 5 V VBUS regulator needs.
Switching to USB OTG mode will be handled by the SMB137C driver
in response to the USB device setting its power supply scope to
"system".

Change-Id: I0ca2216e7587f449731a82e932f8e91e0ea1fb66
Signed-off-by: David Collins <collinsd@codeaurora.org>