drivers: qti-virtual-sensor: Add virtual sensors for QTI chipsets

Virtual sensors are required to define thermal zones to monitor GPU
and CPU Cluster temperatures, which has more than one sensors.

Add a Virtual sensor helper API for QTI chipset drivers to register
virtual thermal zones to estimate the max of all the CPU cluster and
GPU temperatures.

Change-Id: Ifa3a2f93be3128843704e9b6720edef3c6b3155b
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>