dt-bindings: power: supply: qcom,pm8941: use regulator schema for child node

The 'usb-otg-vbus' child node is a regulator so reference the regulator
schema for proper evaluation.

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