PMC-related cleanups

The following short series contains a few patches to abstract and clean up some of the PMC handling on powerpc, followed by the introduction of the sysfs support for PMCs on PA6T.

Greg, patch 3 in this series (the sysdev attribute one) is a prerequisite for the one after. Would you mind ACKing it so Paul can feed it via his tree so we can resolve this dependency?

Thanks,

Olof