selftests: mlxsw: Introduce devlink line card provision/unprovision/activation tests

Introduce basic line card manipulation which consists of provisioning,
unprovisioning and activation of a line card.

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