drm/amdgpu: add nbio callback to query rom offset

Add nbio callback func used to query rom offset.
Used to query the rom offset for fetching the vbios.

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