selftests/powerpc/pmu: Add utility functions to post process the mmap buffer

Add couple of basic utility functions to post process the mmap buffer.
It includes function to read the total number of samples present in the
mmap buffer and function to get the address of the first sample.

Add function "get_intr_regs" which will return pointer to interrupt
registers present in the sample, incase sample type
PERF_SAMPLE_REGS_INTR is set.

Add functions "get_reg_value" which can be used to read any interrupt
register value from a given sample.

Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220127072012.662451-5-kjain@linux.ibm.com