MIPS: Unify perf counter register definitions

Unify definitions for MIPS performance counter register fields in
mipsregs.h rather than duplicating them in perf_events and oprofile.
This will allow future patches to use them to expose performance
counters to KVM guests.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Robert Richter <rric@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: oprofile-list@lists.sf.net
Patchwork: https://patchwork.linux-mips.org/patch/15212/


Signed-off-by: James Hogan <james.hogan@imgtec.com>