arm/dt: msm8610: Add VADC channel node

Add PA therm channel node to support thermal clients.
The channel node can be accessed through both the VADC
and the ADC_TM. Thermal clients access the same node
through ADC_TM.

Change-Id: I6b5b5fc1516ff8c5fd1e6e661086164e3cb4a80e
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>