arch-arm: Update MISCREG_DBGDIDR to point to Armv8 debug arch

Change-Id: I20691ecdaedde6740c706782635b1f9a4491dc51
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/+/70561
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>