arm: extend MiscReg metadata structures

Implement proper handling of RES0/RES1 and RAZ/RAO bitfields.

Change-Id: I344c32c3fb1d142acfb0521ba3590ddd2b1f5360
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/6802
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>