MIPS: Move Cavium CP0 hwrena impl bits to cpu-feature-overrides.h

We had an ugly #ifdef for Cavium Octeon hwrena bits in traps.c, remove
it to mach-cavium-octeon/cpu-feature-overrides.h

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