MIPS: OCTEON: add fixed-link nodes to in-kernel device tree

Currently OCTEON ethernet falls back to phyless operation on
boards where we have no known PHY address or a fixed-link node.
Add fixed-link support for boards that need it, so we can clean up
the platform code and ethernet driver from some legacy code.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org