dt-bindings: olpc_battery: Add XO-1.5 battery

The XO-1 and XO-1.5 batteries apparently differ in an ability to report
ambient temperature.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>