ACPI: button: make real parent for input devices in device tree

Signed-off-by: Andrey Borzenkov <arvidjaar@mail.ru>
Cc: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>