drm/amd/powerplay: support sysfs to get socclk, fclk, dcefclk

Add sys interface to get socclk, fclk and dcefclk for smu.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Gui Chengming <Jack.Gui@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>