cpu/hotplug: Add support for declaring CPU offlining not supported

cpu/hotplug: Add support for declaring CPU offlining not supported

The ACPI MADT mailbox wakeup method doesn't allow to offline a CPU after
it has been woken up.

Currently, offlining is prevented based on the confidential computing attribute
which is set for Intel TDX. But TDX is not the only possible user of the wake up
method. The MADT wakeup can be implemented outside of a confidential computing
environment. Offline support is a property of the wakeup method, not the CoCo
implementation.

Introduce cpu_hotplug_disable_offlining() that can be called to indicate that
CPU offlining should be disabled.

This function is going to replace CC_ATTR_HOTPLUG_DISABLED for ACPI MADT wakeup
method.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Tao Liu <ltao@redhat.com>
Link: https://lore.kernel.org/r/20240614095904.1345461-4-kirill.shutemov@linux.intel.com