MIPS: OCTEON: mark RGMII interface disabled on OCTEON III

Commit 885872b7 ("MIPS: Octeon: Add Octeon III CN7xxx
interface detection") added RGMII interface detection for OCTEON III,
but it results in the following logs:

[    7.165984] ERROR: Unsupported Octeon model in __cvmx_helper_rgmii_probe
[    7.173017] ERROR: Unsupported Octeon model in __cvmx_helper_rgmii_probe

The current RGMII routines are valid only for older OCTEONS that
use GMX/ASX hardware blocks. On later chips AGL should be used,
but support for that is missing in the mainline. Until that is added,
mark the interface as disabled.

Fixes: 885872b7

 ("MIPS: Octeon: Add Octeon III CN7xxx interface detection")
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: stable@vger.kernel.org # 4.7+