net: dsa: add support for DCB get/set apptrust configuration

Add DCB support to get/set trust configuration for different packet
priority information sources. Some switch allow to chose different
source of packet priority classification. For example on KSZ switches it
is possible to configure VLAN PCP and/or DSCP sources.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>