OLPC XO-1.5 ebook switch driver

The OLPC XO-1.5 has an ebook switch, triggered when the laptop
screen is rotated then folding down, converting the device into ebook
form.

This switch is exposed through ACPI. Add a driver that exposes it
to userspace as an input device.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Matthew Garrett <mjg@redhat.com>