MIPS: Allow L2 prefetch to be configured via debugfs

When debugging or examining the performance of a system it can be useful
to examine the effect of L2 prefetching. Provide an optional debugfs
entry to allow a user to enable or disable L2 prefetching.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11182/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>