selftests: mlxsw: Add a scale test for physical ports

Query the maximum number of supported physical ports using devlink-resource
and test that this number can be reached by splitting each of the
splittable ports to its width. Test that an error is returned in case
the maximum number is exceeded.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>