ACPI: dock driver

Create a driver which lives in the acpi subsystem to handle dock events.
This driver is not an "ACPI" driver, because acpi drivers require that the
object be present when the driver is loaded.

Signed-off-by: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Cc: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Len Brown <len.brown@intel.com>