net: dsa: microchip: add KSZ9896 switch support

Add support for the KSZ9896 6-port Gigabit Ethernet Switch to the
ksz9477 driver.

Although the KSZ9896 is already listed in the device tree binding
documentation since a1c0ed24 (dt-bindings: net: dsa: document
additional Microchip KSZ9477 family switches) the chip id
(0x00989600) is not recognized by ksz_switch_detect() and rejected
by the driver.

The KSZ9896 is similar to KSZ9897 but has only one configurable
MII/RMII/RGMII/GMII cpu port.

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