net: dsa: microchip: add KSZ9896 to KSZ9477 I2C driver

Add support for the KSZ9896 6-port Gigabit Ethernet Switch to the
ksz9477 driver. The KSZ9896 supports both SPI (already in) and I2C.

Signed-off-by: Romain Naour <romain.naour@skf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>