net: dsa: microchip: ksz8795: add support for ksz88xx chips

We add support for the ksz8863 and ksz8873 chips which are
using the same register patterns but other offsets as the
ksz8795.

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>