MIPS: perf: Add Octeon support for hardware perf.

Enable hardware counters for Octeon, and add the corresponding event
mappings.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Deng-Cheng Zhu <dengcheng.zhu@gmail.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2790/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>