ARM: dts: msm: Update ADC node for pmxpoorwills

Add property thermal-sensor-cells to register ADC sensors
with of_thermal to allow thermal clients to read these
sensors through thermal zones.

Change-Id: Ia1e2fdd2042b9306e6c52994cd84c5b356941dcc
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>