drm/amdgpu: add UMC to ip discovery map

resolve issue with UMC base offset not being set correctly in ip discovery sequence

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: John Clements <john.clements@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>