net: dsa: microchip: Add Microchip KSZ8863 SPI based driver support

Add KSZ88X3 driver support. We add support for the KXZ88X3 three port
switches using the SPI Interface.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>