On IPQ8064 (and related platforms) the GCC uses PLL4 clock provided by
the LCC clock controller. Mention this in the bindings.

To remain compatible with older bindings, make it optional, as the
driver will fallback to getting the `pll4' clock from the system clocks
list.
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220927113826.246241-2-dmitry.baryshkov@linaro.org