mlxsw: reg: Add Port Prio To Buffer register

When packets ingress the switch they are assigned a switch priority
number that dictates the packet's priority group (PG) buffer in the
port's headroom buffer.

Add the Port Prio To Buffer (PPTB) register, which configures the switch
priority to PG mapping.

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>