arm/dt: Add VADC channel node

Add USB_ID channel node to measure voltage on the
USB_ID line. The USB_ID channel being added uses
a 100k pull-up that is supported by the configurable
internal pull-up of the AMUX arbiter.

Change-Id: I4e45b3a6c45fd8e984dbf87b4c8d8d8328d52b91
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>