hwmon: qpnp-adc: Update thermal zone registration

Switch registering thermal zones with of_thermal to
allow thermal core framework ability to monitor
thermistors for temperature reads.

Change-Id: Ifc11e0d599e82d7912c2d17bb4de0676df2ed401
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>