selftests/powerpc/pmu: Add interface test for bhrb disable field

The testcase uses "instructions" event to generate the
samples and fetch Monitor Mode Control Register A (MMCRA)
when overflow. Branch History Rolling Buffer(bhrb) disable bit
is part of MMCRA which need to be verified by perf interface.
Testcase checks if the bhrb disable bit of MMCRA register is
programmed correctly via perf interface for ISA v3.1 platform
Also make get_mmcra_ifm return type as u64.

Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220610134113.62991-9-atrajeev@linux.vnet.ibm.com