arch-arm: Change disassemble when MSR to UNKNOWN register

arch-arm: Change disassemble when MSR to UNKNOWN register

This patch changes the fault being thrown when MSR/MRS to an unknown
Misc register in AArch64. While previously the instruction was decoded
as an Unknown instruction (hence not printing any information), it is
now decoded as a FailUnimplemented and the unrecognized System register
numbers (CRn, op0...) are printed.

Change-Id: I205ff7adcde5934231c77e8d2250db69a34581fc
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10061
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>