ARM: dts: msm: Add extcon handle for VBUS notification on sdxpoorwills

Add gpio-usbdetect as extcon handle to receive usb device mode vbus
connect/disconnect notification.

Change-Id: I3f33c3406cfe1905e0f17594ebf78f9ea5d67895
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>