selftest/powerpc: Add additional option to mmap_bench test

This patch adds --pgfault and --iterations options to mmap_bench test. With
--pgfault we touch every page mapped. This helps in measuring impact in the
page fault path with a patch series.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>