MIPS: Add LATENCYTOP support

Add LATENCYTOP support for MIPS. Tested on OCTEON.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11353/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>