mlxsw: reg: Add lossless settings for PBMC register

When configuring PAUSE frames and PFC we'll need to configure the
Xon/Xoff threshold for the priority group (PG) buffers.

Add the Xon/Xoff threshold fields to the PBMC register so that we can
configure these when needed.

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>