net: dsa: microchip: add support DSCP priority mapping

Microchip KSZ and LAN variants do not have per port DSCP priority
configuration. Instead there is a global DSCP mapping table.

This patch provides write access to this global DSCP map. In case entry
is "deleted", we map corresponding DSCP entry to a best effort prio,
which is expected to be the default priority for all untagged traffic.

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