MIPS: Octeon: Add sched_clock() to csrc-octeon.c

With the advent of function graph tracing on MIPS, Octeon needs a high
precision sched_clock() implementation.  Without it, most timing
numbers are reported as 0.000.

This new sched_clock just uses the 64-bit cycle counter appropriately
scaled.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Patchwork: http://patchwork.linux-mips.org/patch/805/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>