dt-bindings: net: ethernet-controller: Rework 'fixed-link' schema

While the if/then schemas mostly work, there's a few issues. The 'allOf'
schema will also be true if 'fixed-link' is not an array or object as a
false 'if' schema (without an 'else') will be true. In the array case
doesn't set the type (uint32-array) in the 'then' clause. In the node case,
'additionalProperties' is missing.

Rework the schema to use oneOf with each possible type.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>