dsa: add device tree bindings to register DSA switches

This patch adds support for registering DSA switches using Device Tree
bindings. Note that we support programming the switch routing table even
though no in-tree user seems to require it. I tested this on Armada 370
with a Marvell 88E6172 (not supported by mainline yet).

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>