drivers: base: Implement weak arch_unregister_cpu()

Add arch_unregister_cpu() to allow the ACPI machinery to call
unregister_cpu(). This is enough for arm64, but needs to be
overridden by x86 and ia64 who need to do more work.

CC: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: James Morse <james.morse@arm.com>