arm64: setup: Switch over to GENERIC_CPU_DEVICES using arch_register_cpu()

To allow ACPI's _STA value to hide CPUs that are present, but not
available to online right now due to VMM policy, the register_cpu() call
needs to be made by the ACPI machinery. Systems booting with DT will need
something like we have today.

Switch over to GENERIC_CPU_DEVICES, and provide an arch_register_cpu()
that handles the arch-specific data structure that holds struct cpu.
arch_register_cpu() is also the interface the ACPI machinery expects.
As x86 and ia64 now do this too, the register_cpu() calls can be moved
for systems booting with ACPI in one go.

The memory node register work needs to happen before GENERIC_CPU_DEVICES,
enable GENERIC_MEM_NODES too.

This is an intermediate step to the logic being moved to drivers/acpi,
where GENERIC_CPU_DEVICES will do the work when booting with acpi=off.

Signed-off-by: James Morse <james.morse@arm.com>