powerpc/perf: factor out power8 pmu macros and defines

Factor out some of the power8 pmu macros to
new a header file to share with power9 pmu code.
Just code movement and no logic change.

Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>