dt-bindings: clock: document Amlogic S4 SoC PLL clock controller

Add the S4 PLL clock controller dt-bindings in the S4 SoC family.

Signed-off-by: Yu Tu <yu.tu@amlogic.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230904075504.23263-2-yu.tu@amlogic.com
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>