[Why]
We need to add DP SDP message test debugfs to make sdp message test
more convenient and efficient.

[How]
Add a DM accessible SDP interface for custom data.
Signed-off-by: NLeo (Hanghong) Ma <hanghong.ma@amd.com>
Reviewed-by: NHarry Wentland <Harry.Wentland@amd.com>
Reviewed-by: NRoman Li <Roman.Li@amd.com>
Acked-by: NBhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>