selftests/powerpc/pmu: Add macros to extract mmcr fields

Along with it, Add macros and utility functions to fetch individual
fields from Monitor Mode Control Register 2(MMCR2) register.

Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220127072012.662451-7-kjain@linux.ibm.com