thermal: of: Extend of-thermal.c to provide check if trip point is valid

This patch extends the of-thermal.c to provide check if trip point is
valid.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
(cherry picked from commit a9bf2cc49d9030e374edb9cc0389512a1a1c357e)
Signed-off-by: Kevin Hilman <khilman@linaro.org>