dt-bindings: w1: document bindings for ds2760 battery monitor

This patch adds the devicetree bindings for Maxim's ds2760 battery
monitors.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>