[MIPS] Add debugfs files to show fpuemu statistics

Export contents of struct mips_fpu_emulator_stats via debugfs.

There is no way to read these statistics for now but they (at least
the "emulated" count) might be sometimes useful for performance tuning
on FPU-less CPUs.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>