selftests/powerpc/pmu: Add event_init_sampling function

Extended event_init_opts() to include initialization of sampling
testcases. Patch adds an event_init_sampling() wrapper to initialize
event attribute fields for sampling events. This includes initializing
sample period, sample type and event type.

Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220127072012.662451-6-kjain@linux.ibm.com