net: dsa: Allow multi hop routes to be expressed

With more than two switches in a hierarchy, it becomes necessary to
describe multi-hop routes between switches. The current binding does
not allow this, although the older platform_data did. Extend the link
property to be a list rather than a single phandle to a remote switch.
It is then possible to express that a port should be used to reach
more than one switch and the switch maybe more than one hop away.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>