MIPS: Octeon: Probe for Octeon II CPUs.

The OCTEON II ISA extends the original OCTEON ISA, so give it its own
__elf_platform string so optimized libraries can be selected in
userspace.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Patchwork: http://patchwork.linux-mips.org/patch/1665/


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