ACPI: Add weak routines for ACPI CPU Hotplug

Add weak functions for architectures which do not support
hot-adding and removing CPUs which aren't detected at
bootup. (e.g. via MADT).

This helps preserve the Kconfig dependency from:

commit cbfc1bae ("[ACPI] ACPI_HOTPLUG_CPU Kconfig dependency
update")

    prevent:

    HOTPLUG_CPU=y
    ACPI_PROCESSOR=y
    ACPI_HOTPLUG_CPU=n

Signed-off-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>