MIPS: Octeon: Use non-overflowing arithmetic in sched_clock

With typical mult and shift values, the calculation for Octeon's sched_clock
overflows when using 64-bit arithmetic.  Use 128-bit calculations instead.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/849/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>