Switch arm64 to use the same physmem code as 32-bit arm.

The main advantage of this is to allow us to exclude memory from being
used by the kernel. This may be from the memreserve property, or ranges
marked as no-map under the reserved-memory node.

More work is still needed to remove the physmap array. This is still used
for creating the DMAP region, however other patches need to be committed
before we can remove this.

Obtained from: ABT Systems Ltd
Sponsored by: Turing Robotic Industries