selftests/powerpc/pmu/: Add interface test for mmcr0_pmccext bit

The testcase uses cycles event to check the PMCCEXT bit setting in
Monitor Mode Control Register 0 (MMCR0). Check if perf interface sets
this control bit in MMCR0 on ISA v3.1 platform.

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
[mpe: Add error checking, drop GET_MMCR_FIELD, add to .gitignore]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220127072012.662451-12-kjain@linux.ibm.com