thermal/drivers/core: Use governor table to initialize

Now that the governor table is in place and the macro allows to browse the
table, declare the governor so the entry is added in the governor table
in the init section.

The [un]register_thermal_governors function does no longer need to use the
exported [un]register thermal governor's specific function which in turn
call the [un]register_thermal_governor. The governors are fully
self-encapsulated.

The cyclic dependency is no longer needed, remove it.

Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>