Input: add switch for dock events

Add a SW_DOCK switch to input.h.  ACPI docks currently send their docking
status as a uevent, but not all docks are ACPI or correspond to a device.
In that case, it makes more sense to simply generate an input event on
docking or undocking.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>