dt-bindings: net: mvneta: add phys property

Add an optional phys property to the mvneta binding documentation for
the common phy.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>