selftests/powerpc/pmu: Add selftest for reserved bit check for MMCRA thresh_ctl field

Testcase for reserved bits in Monitor Mode Control Register A (MMCRA)
thresh_ctl bits. For MMCRA[48:51]/[52:55]) Threshold Start/Stop,
0b11110000/0b00001111 is reserved.

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/20220610134113.62991-25-atrajeev@linux.vnet.ibm.com