devlink: introduce possibility to expose info about nested devlinks

In mlx5, there is a devlink instance created for PCI device. Also, one
separate devlink instance is created for auxiliary device that
represents the netdev of uplink port. This relation is currently
invisible to the devlink user.

Benefit from the rel infrastructure and allow for nested devlink
instance to set the relationship for the nested-in devlink instance.
Note that there may be many nested instances, therefore use xarray to
hold the list of rel_indexes for individual nested instances.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>