selftests/powerpc/pmu: Add interface test for extended reg support

The testcase uses check_extended_regs_support and
perf_get_platform_reg_mask function to check if the
platform has extended reg support. This will help to
check if sampling pmu selftest is enabled or not for
a given platform.

Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250113075858.45137-4-atrajeev@linux.vnet.ibm.com