MIPS/Perf-events: Don't do validation on raw events

MIPS licensees may want to modify performance counters to count extra
events. Also, now that the user is working on raw events, the manual is
being used for sure. And feeding unsupported events shouldn't cause
hardware failure and the like.

[ralf@linux-mips.org: performance events also being used in internal
performance evaluation and have a tendency to change as the micro-
architecture evolves, even for minor revisions that may not be
distinguishable by PrID.  It's not very practicable to maintain a list
of all events and there is no real benefit.]

Signed-off-by: Deng-Cheng Zhu <dczhu@mips.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@ghostprotocols.net>
Cc: David Daney <david.daney@cavium.com>
Cc: Eyal Barzilay <eyal@mips.com>
Cc: Zenon Fortuna <zenon@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/3107/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>