arch-arm: Make the Tarmac parsed registers case insensitive

arch-arm: Make the Tarmac parsed registers case insensitive

This will make parsing more robust, considering the tarmac
format changes between AA32 and AA64.

Change-Id: I0e4905d70e2e494104706a4c6c75b8169deaecf9
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22845
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>