dt-bindings: net: dsa-port: constrain number of 'reg' in ports

'reg' without any constraints allows multiple items which is not the
intention in DSA port schema (as physical port is expected to have only
one address).

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>