arm: Clean up the GIC implementation

arm: Clean up the GIC implementation

Lots of minor cleaups:
  * Make cached params const
  * Don't serialize params
  * Use AddrRange to represent the distributor and CPU address spaces
  * Store a const AddrRangeList of all PIO ranges

Change-Id: I40a17bc3a38868fb3b8af247790e852cf99ddf1d
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2330
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>