thermal: Introduce dummy functions when thermal is not defined

When CONFIG_THERMAL is not enabled, it is better to introduce
equivalent dummy functions in the exported header than to
introduce #ifdeffery in drivers using the function.

This will prevent issues such as that reported in:
http://www.spinics.net/lists/linux-next/msg31573.html



While at it switch over to IS_ENABLED for thermal macros
to allow for thermal framework to be built as framework
and relevant APIs be usable by relevant drivers as a result.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>