drm/amdgpu: mention RDNA support in docu

The amdgpu kernel module has supported RDNA for a while,
mention that in the module description.

v2: Add CDNA as well (Alex)

Signed-off-by: Peter Maucher <bellosilicio@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>