drivers: thermal: Use deferrable work and power efficient workqueue

Thermal core uses work events to poll for sensor driver temperature
crossing a threshold. Since it is not using a deferrable workqueue, it
might wake-up the device from sleep.

Use a deferrable work event and post the work in the power efficient
workqueue for estimating virtual sensor temperature.

Change-Id: I9dd21d8fc4e5ca96e06db9ecb57a628618494a01
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>