drm/amdgpu: add OSS 2.4 register headers

These are register headers for the OSS (OS Services)
block on the GPU.

Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Jammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>