mlxsw: spectrum_port_range: Add devlink resource support

Expose via devlink-resource the maximum number of port range registers
and their current occupancy. Besides the observability benefits, this
resource will be used by subsequent patches for scale and occupancy
tests.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/7945e0c715dc5efb1617f45f7560c1f1bd0bcf8a.1689092769.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>