net: dsa: Provide a slave MII bus if needed

Mimic what net/dsa/dsa.c does and provide a slave MII bus by default
which will be created if the driver implements a phy_read method.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>