ARM: dts: msm: Add vbus regulator handle for host mode on sdxpoorwills

Vbus regulator is turned on by usb dwc3 driver as part of starting host
mode.

Change-Id: I68e6b1e9ce96ee367422822b6bb8a2f0a7a3d3a8
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>