Add a simple documentation in the dmub_cmd.
Tested-by: NMark Broadworth <mark.broadworth@amd.com>
Reviewed-by: NAric Cyr <Aric.Cyr@amd.com>
Acked-by: NRodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: NAnthony Koo <Anthony.Koo@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>