thermal: Move internal IPA functions

The exported IPA functions are used by the IPA. It is pointless to
declare the functions in the thermal.h file.

For better self-encapsulation and less impact for the compilation if a
change is made on it. Move the code in the thermal core internal
header file.

As the users depends on THERMAL then it is pointless to have the stub,
remove them.

Take also the opportunity to fix checkpatch warnings/errors when
moving the code around.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20200402142747.8307-3-daniel.lezcano@linaro.org