drm/amdgpu: Expose hive adev list and xgmi_mutex

It's needed for device reset of entire hive.

v3:
Add per hive lock to allow avoiding duplicate resets triggered by
multiple members  of same hive.
Expose amdgpu_hive_info instead of adding getter functions.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>