MIPS: Don't save/restore OCTEON wide multiplier state on syscalls.

The ABI allows these to be clobbered on syscalls, so only save and
restore the multiplier state when the temporary registers need to be
preserved.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: David Daney <david.daney@cavium.com>
Patchwork: https://patchwork.linux-mips.org/patch/5540/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>