net: lan966x: Add support for PCP rewrite

Add support for rewrite of PCP and DEI value, based on QoS and DP level.

The DCB rewrite table is queried for mappings between priority and
PCP/DEI. The classified DP level is then encoded in the DEI bit, if a
mapping for DEI exists.

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>