selftests/powerpc/pmu/: Add interface test for mmcr0_pmc56 using pmc5

The testcase uses event code 0x500fa to verify the FC5-6 bit setting in
Monitor Mode Control Register 0 (MMCR0). Check if FC5-6 bit is not set
in MMCR0 when using Performance Monitor Counter 5 and 6 (PMC5 and PMC6).

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/20220127072012.662451-15-kjain@linux.ibm.com