thermal: add a warning for temperature emulation feature

Because this feature is for debuging purposes, it is highly
recommended to do not enable this on production systems.
This patch adds warnings for system integrators, so that
people are aware of this potential security issue.

Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>