arch-arm: Move MiscReg BitUnions into a separate header file

arch-arm: Move MiscReg BitUnions into a separate header file

As the number of MiscRegs increases there is the need of a cleaner
header file; this patch is separating miscreg.[cc,hh] where the miscreg
decoding/utility functions are defined/declared from the MiscReg
register types (BitUnion).

Change-Id: I815f53b117b50e47f93aa13d78356ef09d2a2541
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/13065
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>