drm/amdgpu: add structures for rlcg indirect reg access

Add structures that are used to cache registers offsets
for rlcg indirect reg access ctrl and flag availability
of such interface

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Zhou, Peng Ju <PengJu.Zhou@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>