mem-cache: Fix secure bit modification

mem-cache: Fix secure bit modification

Secure bit was being updated outside insertion.

Change-Id: I83d9b010e8cf64013bbea9bae3ea68b0c414a189
Reviewed-on: https://gem5-review.googlesource.com/10622
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>