arch-arm: Change function name for banked miscregs

arch-arm: Change function name for banked miscregs

This commit changes the function's name used for retrieving the index of a
security banked register given the flatten index. This will avoid confusion
with flattenRegId, which has a different purpose.

Change-Id: I470ffb55916cb7fc9f78e071a7f2e609c1829f1a
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/7982
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>