selftests/powerpc/pmu: Add selftest for mmcr1 pmcxsel/unit/cache fields

The testcase uses event code "0x21c040" to verify the settings for
different fields in Monitor Mode Control Register 1 (MMCR1). The fields
include PMCxSEL, PMCXCOMB PMCxUNIT, cache. Checks if these fields are
translated correctly via perf interface to MMCR1

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/20220610134113.62991-14-atrajeev@linux.vnet.ibm.com