Build: Improve condition of including ps_nv_counters.c

Roll-back protection test cases are only selected in PS secure
regression test suites. Remove the dependency of TEST_NS from
condition of including ps_nv_counters.c in PS secure partition.

Update the comment and the document accordingly.

Change-Id: I443748dffd1e920329a0d4478b57d19163c707db
Signed-off-by: David Hu <david.hu@arm.com>
2 files changed