arch-arm: Reduce boilerplate when extracting SelfDebug from tc

arch-arm: Reduce boilerplate when extracting SelfDebug from tc

Change-Id: I1746400617be64ac9c2f3194442734e178342909
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31354
Tested-by: kokoro <noreply+kokoro@google.com>