mlxsw: spectrum: Support for counters on router interfaces

Add support for counter allocation on router interfaces. The allocation
depends on the counter state of relevant table. In case the counting is
disabled or no counters left the counter index will be set as invalid.

Also a counter pool for router allocation is added.

Signed-off-by: Arakdi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>