mlxsw: spectrum: Add support for counter allocator

Add implementation for counter allocator. The ASIC has special memory
pool for various counting purposes. Counter memory is distributed between
equal size banks.

The static sub-pool configuration should specify the following parameters
for each sub-pool:
- Number of required banks.
- Maximum entry size.

Each module can add dedicated sub-pool or use existing one.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>