dt-bindings: net: Add fixed-link support

Update device-tree binding with fixed-link support.

With fixed-link support the formerly required property 'phy-handle'
is now optional if 'fixed-link' child is present.

Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>