drivers: thermal: of-thermal: Add support for virtual sensor

of-thermal assumes that there will be only one sensor per thermal zone.
But there can be multiple sensors in the thermal zone and the
temperatures can be aggregated together.

Add support for of-thermal to take multiple sensors per thermal zone and
provide three ways for aggregating the sensor temperatures. The
temperatures can be aggregated based on coefficients or get the maximum of
all temperatures or get minimum of all temperatures.

Change-Id: Id30fe6f437bbd916aa89963c4be983b27dbe23ff
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>