dt-bindings: power: supply: qcom,pm8941: document usb-charge-current-limit

Document already used (in DTS and by driver) 'usb-charge-current-limit'
property:

  arch/arm/boot/dts/qcom-msm8974pro-fairphone-fp2.dtb:
    charger@1000: 'usb-charge-current-limit' does not match any of the regexes: 'pinctrl-[0-9]+'

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>