drm/amd/powerplay: implement interface to retrieve clock freq for renoir

implement smu12 get_clk_freq interface to get clock frequency like
MCLK/SCLK.

Signed-off-by: Xiaomeng Hou <Xiaomeng.Hou@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>