net: dsa: microchip: add support for different DCB app configurations

Add DCB support to configure app trust sources and default port priority.

Following commands can be used for testing:
dcb apptrust set dev lan1 order pcp dscp
dcb app replace dev lan1 default-prio 3

Since it is not possible to configure DSCP-Prio mapping per port, this
patch provide only ability to read switch global dscp-prio mapping and
way to enable/disable app trust for DSCP.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>