Add capability for the user to clear all FP emu debugfs counters.

This is achieved by having a special debugfs file "fpuemustats_clear"
(under default location "/sys/kernel/debug/mips"). Each access to the
file results in setting all counters to zero (it is enough, let's say,
to issue a "cat /sys/kernel/debug/mips/fpuemustats_clear").

This functionality already exists for R2 emulation statistics,
but was missing for FP emulation statistics. The implementation in
this patch is consistent with its R2 emulation counterpart.
Signed-off-by: Miodrag Dinic <miodrag.dinic@imgtec.com>
Signed-off-by: Goran Ferenc <goran.ferenc@imgtec.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@imgtec.com>
Cc: Douglas Leung <douglas.leung@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Petar Jovanovic <petar.jovanovic@imgtec.com>
Cc: Raghu Gandham <raghu.gandham@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17144/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>