MIPS: Octeon: Add device tree source files.

The two device tree files octeon_3xxx.dts and octeon_68xx.dts are
trimmed by code in a subsequent patch to reflect the hardware actually
present on the board. To this end several properties that are not
part of the declared bindings are added to aid in trimming off
unwanted nodes. Since the device tree and the code that trims it are
bound into the kernel binary, these 'marker' properties never escape
into the wild, and are purely an implementation detail of the kernel
early boot process. This is done for backwards compatibility with
existing boards (identified by a board type enumeration value by their
bootloaders). New boards will always pass a device tree from the
bootloader, the built-in trees are ignored in this case.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: devicetree-discuss@lists.ozlabs.org
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: linux-kernel@vger.kernel.org
Cc: David Daney <david.daney@cavium.com>
Patchwork: https://patchwork.linux-mips.org/patch/3937/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>