net: lan966x: Add support for DSCP rewrite

Add support for DSCP rewrite in lan966x driver. On egress DSCP is
rewritten from either classified DSCP, or frame DSCP. Classified DSCP is
determined by the Analyzer Classifier on ingress, and is mapped from
classified QoS class and DP level. Classification of DSCP is by default
enabled for all ports.

It is required that DSCP is trusted for the egress port *and* rewrite
table is not empty, in order to rewrite DSCP based on classified DSCP,
otherwise DSCP is always rewritten from frame DSCP.

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>