MIPS: OCTEON: Enable DEVTMPFS

Recent versions of udev and systemd require the kernel to be compiled
with CONFIG_DEVTMPFS in order to populate the /dev directory. Most MIPS
platforms have it enabled by default, so enable it for the Cavium Octeon
defconfig as well. This will assist with automated kernel boot testing.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: David Daney <david.daney@cavium.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15294/


Signed-off-by: James Hogan <james.hogan@imgtec.com>