net: lan966x: Add support for offloading dscp table

Add support for offloading dscp app entries. The dscp values are global
for all lan966x ports.

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