mlxsw: reg: Add QoS Port DSCP to Priority Mapping Register

Add QPDP. This register controls the port default Switch Priority and
Color. The default Switch Priority and Color are used for frames where the
trust state uses default values. Currently there are two cases where this
applies: a port is in trust-PCP state, but a packet arrives untagged; and a
port is in trust-DSCP state, but a non-IP packet arrives.

Signed-off-by: Petr Machata <petrm@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>