dt-bindings: net: dsa: qca8k: fix example

In the example, the phy at phy@0 is clashing with
the switch0@0 at the same address. Usually, the switches
are accessible through pseudo PHYs which in case of the
qca8k are located at 0x10 - 0x18.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>