dt-bindings: power: supply: define monitored-battery in common place

Define the type of monitored-battery in power-supply.yaml common schema.
Reference the schema where applicable to enforce the above in bindings
which have monitored-battery property.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>