drivers: thermal: Add devfreq vdd floor cooling device

This implements a mitigation device to place a minimum frequency floor
on a particular devfreq device. This mitigation device will be used by
low temperature reliability rules to mitigate a devfreq device at
minimum operating level.

Change-Id: Iaae6ea4ffa12735145fc2b573ddfa98183b379fe
Signed-off-by: Manaf Meethalavalappu Pallikunhi <manafm@codeaurora.org>
Signed-off-by: Priyansh Jain <priyjain@codeaurora.org>