thermal: core: Change governor name to const char pointer

All users are already assigning a const char * to the `governor_name`
member of struct thermal_zone_params and to the `name` member of
struct thermal_governor.
Even if users are technically wrong, it just makes more sense to change
this member to be a const char pointer instead of doing the other way
around.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>