MIPS: Octeon: Adding driver to measure interrupt latency on Octeon.

Signed-off-by: Venkat Subbiah <venkat.subbiah@cavium.com>
[Rewrote timeing calculations]
Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4660/


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