net: dsa: microchip: Add support for bridge port isolation

Implement bridge port isolation for KSZ switches. Enabling the isolation
of switch ports from each other while maintaining connectivity with the
CPU and other forwarding ports. For instance, to isolate swp1 and swp2
from each other, use the following commands:
- bridge link set dev swp1 isolated on
- bridge link set dev swp2 isolated on

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>