arch-arm: Refactor AArch64 MSR/MRS trapping

arch-arm: Refactor AArch64 MSR/MRS trapping

This patch refactors AArch64 MSR/MRS trapping, by moving the trapping
helpers in arch/arm/utility and in the isa code into a MiscRegOp64
class.

This class is the Base class for a generic AArch64 instruction which is
making use of system registers (MiscReg), like MSR,MRS,SYS.  The common
denominator or those instruction is the chance that the system register
access is trapped to an upper Exception level. MiscRegOp64 is providing
that feature.

What do we gain? Other "pseudo" instructions, like access to
implementation defined registers can inherit from this class to make use
of the trapping functionalities even if there is no data movement
between GPRs and system register.

Change-Id: I0924354db100de04f1079a1ab43d4fd32039e08d
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/13778
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>