thermal: Introduce new property monitor_type for trip point.

Thermal trip points can be defined to indicate whether a
temperature rise or a temperature fall is to be monitored. This
property can now be defined in the DT bindings for a trip point.
To support this following three changes are introduced to thermal
core and sysfs code.
1. Define a new variable in thermal_trip to capture the monitor
   rising/falling information from trip point DT bindings.
2. Define a new ops in thermal_zone_device_ops that can be populated
   to indicate whether a trip is being monitored for rising or falling
   temperature. If the ops is not populated or if the binding is missing
   in the DT, it is assumed that the trip is being monitored for rising
   temperature. (default behavior today)

Signed-off-by: Thara Gopinath <thara.gopinath@linaro.org>