selftests/powerpc/pmu: Add interface test for mmcra_thresh_cmp fields

The testcase uses event code 0x35340401e0 for load only sampling, to
verify the settings of thresh compare field in Monitor Mode Control
Register A (MMCRA: 9-18 bits for power9 and MMCRA: 8-18 bits for
power10). Testcase checks if the thresh compare field is programmed
correctly via perf interface to MMCRA register.

Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220610134113.62991-4-atrajeev@linux.vnet.ibm.com