dsa: add DSA switch driver for Microchip KSZ9477

The KSZ9477 is a fully integrated layer 2, managed, 7 ports GigE switch
with numerous advanced features. 5 ports incorporate 10/100/1000 Mbps PHYs.
The other 2 ports have interfaces that can be configured as SGMII, RGMII, MII
or RMII. Either of these may connect directly to a host processor or
to an external PHY. The SGMII port may interface to a fiber optic transceiver.

This driver currently supports vlan, fdb, mdb & mirror dsa switch operations.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Woojung Huh <Woojung.Huh@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>