mlxsw: Add a resource describing number of RIFs

The Spectrum ASIC has a limit on how many L3 devices (called RIFs) can be
created. The limit depends on the ASIC and FW revision, and mlxsw reads it
from the FW. In order to communicate both the number of RIFs that there can
be, and how many are taken now (i.e. occupancy), introduce a corresponding
devlink resource.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>