Support for ARM GIC 500

Some ARMv8 cores do not have a GIC that has backwards
compatibility. This adds ARM GIC 500 (GICv3 and GICv4) support to
seL4. It should also be noted that there are much more distributor and
redistributor registers than in previous version. The platform
implementor that needs a GIC500 should take care to set up the kernel
devices properly.

Change-Id: Ia7c546f7874a758ecd1ee8a29dd749eb3a2444f3