selftests/powerpc/pmu: Include mmap_buffer field as part of struct event

To enable the capturing of samples as part of perf event, add a new
field "mmap_buffer" to "struct event". This field is a place-holder for
sample collection

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220127072012.662451-2-kjain@linux.ibm.com