drm/amdgpu/include: Add sdma0/1 4.2 register headerfiles. (v3)

These are the System DMA register headers for vega20.

v2: cleanups (Alex)
v3: add missing licenses (Alex)

Signed-off-by: Feifei Xu <Feifei.Xu@amd.com>
Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>