selftests: microchip: add test for QoS support on KSZ9477 switch family

Add tests covering following functionality on KSZ9477 switch family:
- default port priority
- global DSCP to Internal Priority Mapping
- apptrust configuration

This script was tested on KSZ9893R

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