dt-bindings: net: remove reference to fixed link support

Contrary to what the Device Tree binding indicates, the binding for the
PPv2 network device currently doesn't provide any fixed link
functionality. This commit adjusts the Device Tree binding documentation
accordingly.

Reported-by: Russell King - ARM Linux <linux@armlinux.org.uk>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Rob Herring <robh@kernel.org>