powerpc: Correct annotations of pmu registration functions

This fixes the following warning:
WARNING: arch/powerpc/kernel/built-in.o(.text+0x29768): Section mismatch in reference from the function .register_power_pmu() to the function .cpuinit.text:.power_pmu_notifier()
The function .register_power_pmu() references
the function __cpuinit .power_pmu_notifier().
This is often because .register_power_pmu lacks a __cpuinit
annotation or the annotation of .power_pmu_notifier is wrong.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>