Add basic CN7XXX interface detection.

This allows the kernel to boot with ethernet working as it initializes
the ethernet ports with SGMII instead of defaulting to RGMII routines.

Tested on the utm8 from Rhino Labs with a CN7130.
Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Acked-by: David Daney <david.daney@cavium.com>
Cc: janne.huttunen@nokia.com
Cc: aaro.koskinen@nokia.com
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12376/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>