arm64: dts: qcom: sc8280xp: drop reference-clock source

The source clock for the reference clock should not be described by the
devicetree binding and instead this relationship should be modelled in
the clock driver.

Update the USB PHY nodes to match the fixed binding.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221111093857.11360-4-johan+linaro@kernel.org