drm/amd/powerplay: implement sysfs for getting dpm clock

With the common interface print_clk_levels can get the following dpm clock:

-pp_dpm_dcefclk
-pp_dpm_fclk
-pp_dpm_mclk
-pp_dpm_sclk
-pp_dpm_socclk

Signed-off-by: Prike Liang <Prike.Liang@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>