mlxsw: reg: Introduce per priority counters

We are going to add support for PFC as part of DCB ops, which requires us
to report the number of PFC frames sent and received per priority.

Add per priority counters in order to report number of PFC frames sent
and received per priority.

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