thermal: Move default governor config option to the internal header

The default governor set at compilation time is a thermal internal
business, no need to export to the global thermal header.

Move the config options to the internal header.

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/20200402142747.8307-1-daniel.lezcano@linaro.org