dt-bindings: power: supply: cpcap-battery: Add power-supplies property

The binding for cpcap-battery is missing the standard power-supplies
property as noted by Sebastian Reichel <sebastian.reichel@collabora.co.uk>.

Cc: devicetree@vger.kernel.org
Cc: Marcel Partap <mpartap@gmx.net>
Cc: Michael Scott <michael.scott@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>