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

'reg' without any constraints allows multiple items which is not the
intention for Ethernet controller's port number.

Constrain the 'reg' on AX88178 and LAN95xx USB Ethernet Controllers.

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