drm/amdgpu: add helpers to access registers on different AIDs

SMN address which is larger than 32bit has different indications
through bit[34:32] on different AIDs.

v2: put smn addressing of different AIDs into asic specific place
v3: change to ext_id/ext_offset naming

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>