MIPS: Octeon: Add DTS for D-Link DSR-1000N

Add DTS for D-Link DSR-1000N that is usable as is without any "pruning"
with APPENDED_DTB. Split out the common parts from octeon_3xxx.dts
into octeon_3xxx.dtsi.

Compared to builtin generic DTB, we can specificy fixed links properly
and avoid probing non-existent I2C devices.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: David Daney <ddaney.cavm@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12840/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>