selftests/powerpc/pmu: Add support for perf sampling tests

Add support functions for enabling perf sampling test in a new folder
"sampling_tests" under "selftests/powerpc/pmu". This includes support
functions for allocating and processing the mmap buffer. These functions
are added/defined in "sampling_tests/misc.*" files.

Also updates the corresponding Makefiles in "selftests/powerpc" and
"sampling_tests" folder.

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
[mpe: Drop unneeded bits from the Makefile]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220127072012.662451-3-kjain@linux.ibm.com