arch-arm: Print register name when warning on AT instructions

arch-arm: Print register name when warning on AT instructions

With this patch we don't print anymore the enum value when warning
on any AT instruction usage, but we print the sys reg name instead

Change-Id: Ic6831b98ad6b0582be69883dc9519a9d3324da30
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Anouk Van Laer <anouk.vanlaer@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/14316
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>