powerpc: Estimate G5 cpufreq transition latency

Setting G5's cpu frequency transition latency to CPUFREQ_ETERNAL stops
ondemand governor from working. I measured the latency using sched_clock
and haven't seen much higher than 11000ns, so I set this to 12000ns for
my configuration. Possibly other configurations will be different?
Ideally the generic code would be able to measure it in case the platform
does not provide it.

But this simple patch at least makes it throttle again.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>