x86/geode/alix2: Supplement driver to include GPIO button support

GPIO 24 is used in reference designs as a soft-reset button, and
the alix2 is no exception.  Add it as a gpio-button.

Use symbolic values to describe BIOS addresses.

Record the model number.

Signed-off-by: Philip A. Prindeville <philipp@redfish-solutions.com>
Acked-by: Ed Wildgoose <kernel@wildgooses.com>
Acked-by: Andres Salomon <dilinger@queued.net>
Cc: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/n/tip-sjp6k1rjksitx1pej0c0qxd1@git.kernel.org
[ tidied up the code a bit ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>