net: dsa: add support switches global DSCP priority mapping

Some switches like Microchip KSZ variants do not support per port DSCP
priority configuration. Instead there is a global DSCP mapping table.

To handle it, we will accept set/del request to any of user ports to
make global configuration and update dcb app entries for all other
ports.

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