dt-bindings: power: sc27xx: Add one IIO channel to read charge voltage

Add one IIO channel named "charge_vol" to read the charge voltage for
the SC27XX fuel gauge controller.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>