selftests: mlxsw: Add tc flower scale test

Add test of capacity to offload flower.

This is a generic portion of the test that is meant to be called from a
driver that supplies a particular number of rules to be tested with.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>