MIPS: Add user stack and registers to perf.

MIPS: Add user stack and registers to perf.

This allows for extracting off-line stack traces from user-space code
in the perf tool.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Patchwork: https://patchwork.linux-mips.org/patch/5246/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>