arch-arm: understandably initialize register permissions

arch-arm: understandably initialize register permissions

Move massive initialization routine to the bottom of miscregs.cc.
Additionally, share register metadata across ISA instances by
making lookUpMiscReg a static member of the ISA and only
initializing it once.

Change-Id: I6d6ab26200c4e781151cc6efd97ce2420e2bf4cc
Signed-off-by: Curtis Dunham <Curtis.Dunham@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/6803
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>