dt-bindings: phy: qcom,qmp-usb: add missing child node schema

Add the missing the description of the PHY-provider child node which was
ignored when converting to DT schema.

Fixes: ccf51c1c

 ("dt-bindings: phy: qcom,qmp: Convert QMP PHY bindings to yaml")
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20220830112923.3725-21-johan+linaro@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>