powerpc/power8: Fix oprofile and perf

In 2ac6f427

 powerpc/cputable: Fix oprofile_cpu_type on power8
we broke all power8 hw events.

This reverts this change and uses oprofile_type instead. Perf now works
on POWER8 again and oprofile will revert to using timers on POWER8.

Kudos to mpe this fix.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>