net: add IEEE 802.1q specific helpers

IEEE 802.1q specification provides recommendation and examples which can
be used as good default values for different drivers.

This patch implements mapping examples documented in IEEE 802.1Q-2022 in
Annex I "I.3 Traffic type to traffic class mapping" and IETF DSCP naming
and mapping DSCP to Traffic Type inspired by RFC8325.

This helpers will be used in followup patches for dsa/microchip DCB
implementation.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>