net: dsa: microchip: break KSZ9477 DSA driver into two files

Break KSZ9477 DSA driver into two files in preparation to add more KSZ
switch drivers.
Add common functions in ksz_common.h so that other KSZ switch drivers
can access code in ksz_common.c.
Add ksz_spi.h for common functions used by KSZ switch SPI drivers.

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