mlxsw: reg: Add QoS Switch Traffic Class Table register

As part of DCB ops we'll have to configure the priority to traffic class
mapping of a port.

Add the QoS Switch Traffic Class Table (QTCT) register, which configures
the mapping between the packet switch priority and traffic class on the
transmit port.

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>