thermal: of: Extend of-thermal to export table of trip points

This patch extends the of-thermal.c to export trip points for a given
thermal zone.

Thermal drivers should use of_thermal_get_trip_points() method to get
pointer to table of thermal trip points.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
(cherry picked from commit ce8be7785922de0ef497b20384425ed04f674f9d)
Signed-off-by: Kevin Hilman <khilman@linaro.org>