net: dsa: microchip: add KSZ9893 switch support

Add KSZ9893 switch support in KSZ9477 driver.  This switch is similar to
KSZ9477 except the ingress tail tag has 1 byte instead of 2 bytes, so
KSZ9893 tagging will be used.

The XMII register that governs how the host port communicates with the
MAC also has different register definitions.

Signed-off-by: Tristram Ha <Tristram.Ha@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>