net: lan966x: Add registers to configure PCP, DEI, DSCP

Add the registers that are needed to configure the PCP, DEI and DSCP
of the switch both at ingress and also at egress.

Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Piotr Raczynski <piotr.raczynski@intel.com>
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>