add get_power_limit and set_power_limit functions
to support hwmon for SMU11.

v2: fix the code style issue.
Signed-off-by: Chengming Gui <Jack.Gui@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>