ACPI / hotplug / PCI: Define hotplug context lock in the core

Subsequent changes will require the ACPI core to acquire the lock
protecting the ACPIPHP hotplug contexts, so move the definition of
the lock to the core and change its name to be more generic.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>