ia64/topology: Switch over to GENERIC_CPU_DEVICES and GENERIC_MEM_NODE

ia64 has its own arch specific data structure for cpus struct ia64_cpu.
But this has one member, making ia64's cpu_devices the same as
GENERIC_CPU_DEVICES. Only, ia64's cpu_devices has no users, instead the
sysfs_cpus allocated at boot is used.

Remove the arch specific structure allocation and initialisation.
ia64's arch_register_cpu() now overrides the weak version from
GENERIC_CPU_DEVICES to provide the same information.

The memory node register work needs to happen before GENERIC_CPU_DEVICES,
enable GENERIC_MEM_NODE 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>