MIPS: Hook up Cavium OCTEON in arch/mips.

Take all the OCTEON specific files that were added, and hook them into
the build system for the arch/mips.  For versions of GCC that lack
OCTEON support, override gas target architecture.

Signed-off-by: Tomaso Paoletti <tpaoletti@caviumnetworks.com>
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>