drivers: of-thermal: Add support for thermal governor config

Different subsystems in a device require different thermal mitigation
strategies. So using one governor for all the thermal zones will not be
ideal in those cases and there is a need for using different governors to
monitor different thermal zones.

Add support for specifying the thermal governor to be used for a thermal
zone in the devicetree. The devicetree config should specify the
governor name as a string that matches any available governors. The
default governor will be used if the governor name is not specified.

Change-Id: I190194943a93ab6eb5ed5218ec16845db93498b8
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>