dt-bindings: net: ave: add syscon-phy-mode property to configure phy-mode setting

Add "socionext,syscon-phy-mode" property to specify system controller that
configures the settings about phy-mode.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>