drivers: thermal: Add virtual thermal zone

SoCs can have multiple thermal sensors monitoring different hot spots of
a same subsystem. In those cases both the sensor temperatures should be
considered for monitoring and mitigation.

In order to support such cases, add support for virtual thermal zone
interface. These virtual thermal zone will use the dependent sensors
thermal zone to get the temperature and aggregate the different sensors
temperatures. The sensors to be used and the way temperatures are
aggregated has to be defined during the virtual zone registration.
Since this is virtual thermal zone, this sensor requires
the thermal core to poll to monitor the trip temperatures.

The current interface provides three aggregation mechanism. It can
either find the,
1. maximum of all sensor readings
2. minimum of all sensor readings
3. weighted aggregation of sensor readings

Change-Id: I7f38a3612ff56de9e7a56a9a337da87823b65bbc
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>