mlxsw: spectrum_buffers: Add the ability to query the CPU port's shared buffer

While debugging packet loss towards the CPU, it is useful to be able to
query the CPU port's shared buffer quotas and occupancy.

Since the CPU port has no ingress buffers, all the shared buffers ingress
information will be cleared.

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>