selftests/powerpc/pmu: Add support for branch sampling in get_intr_regs function

Add support for sample type as PERF_SAMPLE_BRANCH_STACK in sampling
tests. This change is a precursor/helper for sampling testcases, that
test branck stack feature in perf interface.

Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220610134113.62991-5-atrajeev@linux.vnet.ibm.com