perfcounters/powerpc: add support for POWER4 processors

Impact: more hardware support

This adds the back-end for the PMU on the POWER4 and POWER4+ processors
(GP and GQ).  This is quite similar to the PPC970, with 8 PMCs, but has
fewer events than the PPC970.

Signed-off-by: Paul Mackerras <paulus@samba.org>