thermal: Move struct thermal_attr to the private header

The structure belongs to the thermal core internals but it is exported
in the include/linux/thermal.h

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

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/20200402142747.8307-2-daniel.lezcano@linaro.org