ACPI / hotplug / PCI: Rename register_slot() to acpiphp_add_context()

The name of register_slot() doesn't really reflect what the function
is does, so rename it to acpiphp_add_context() and add a proper
kerneldoc comment to it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>