Thermal: rename thermal governor Kconfig option to avoid generic naming

Currently, we have three Kconfig options for thermal governors, aka,
CONFIG_FAIR_SHARE, CONFIG_USER_SPACE and CONFIG_STEP_WISE.
But these names are too generic that may bring confusion to users.

Rename them to CONFIG_THERMAL_GOV_FAIR_SHARE,
CONFIG_THERMAL_GOV_USER_SPACE, CONFIG_THERMAL_GOV_STEP_WISE
to avoid the generic naming.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>