mlxsw: reg: Add Port Flow Control Configuration register

Add the Port Flow Control Configuration (PFCC) register, which
configures both flow control and Priority-based Flow Control (PFC).

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>