mlxsw: reg: Add Port Physical Loopback Register

The PPLR register allows configuration of the port's loopback mode.

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