powerpc/perf: Update raw-event code encoding comment for power8

Comment explanning the raw event code encoding for Power8 was
moved to isa207_common.h file when re-factoring the code to
support power9. But then Power9 pmu branched out due to changes
specific to power9. So move the encoding comment back to power8-pmu.c
Just comment movement and no logic change.

Fixes: 4d3576b2 ('powerpc/perf: factor out power8 pmu macros and defines')
Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>