dsa: Support multiple MDIO busses

When using a cluster of switches, some topologies will have an MDIO
bus per switch, not one for the whole cluster. Allow this to be
represented in the device tree, by adding an optional mii-bus property
at the switch level. The old platform_device method of instantiation
supports this already, so only the device tree binding needs extending
with an additional optional phandle.

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