thermal: gov_step_wise: Extend thermal step-wise governor to monitor falling temperature.

From the step wise governor point of view, the policy decisions
that has to taken on a thermal trip point that is defined to be monitored
for falling temperature is the mirror opposite of the decisions it has
to take on a trip point that is monitored for rising temperature.

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