selftests/powerpc/pmu: Add support for perf event code tests

Add new folder for enabling perf event code tests which includes
checking for group constraints, valid/invalid events, also checks for
event excludes, alternatives so on. A new folder "event_code_tests", is
created under "selftests/powerpc/pmu".

Also updates the corresponding Makefiles in "selftests/powerpc" and
"event_code_tests" folder.

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/20220610134113.62991-16-atrajeev@linux.vnet.ibm.com