ACPI: Add support for acpi_load_table/acpi_unload_table_id

Make acpi_load_table() available for use by removing it from the #ifdef
ACPI_FUTURE_USAGE.

Also add a new routine used to unload an ACPI table of a given type and "id" -
acpi_unload_table_id().  The implementation of this new routine was almost a
direct copy of existing routine acpi_unload_table() - only difference being
that it only removes a specific table id instead of ALL tables of a given
type.  The SN hotplug driver (sgi_hotplug.c) now uses both of these interfaces
to dynamically load and unload SSDT ACPI tables.

Also, a few other ACPI routines now used by the SN hotplug driver are exported
(since the driver can be a loadable module):

 acpi_ns_map_handle_to_node
 acpi_ns_convert_entry_to_handle
 acpi_ns_get_next_node

Signed-off-by: Aaron Young <ayoung@sgi.com>
Cc: Greg KH <greg@kroah.com>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Len Brown <len.brown@intel.com>