dsa: fix device tree binding documentation typo on #address-cells

The device tree binding documentation for dsa explicitely states that a
DSA node should have its #address-cells property set to 2, yet the
example still used 1, fix that typo.

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