x86, olpc: Add XO-1 poweroff support

Add a pm_power_off handler for the OLPC XO-1 laptop.

The driver can be built modular and follows the behaviour of the
APM driver, setting pm_power_off to NULL on unload. However, the
ability to unload the module will probably be removed (with a simple
__module_get(THIS_MODULE)) if/when XO-1 suspend/resume support is
added to this file at a later date.

Signed-off-by: Daniel Drake <dsd@laptop.org>
LKML-Reference: <20101010094032.9AE669D401B@zog.reactivated.net>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>