This add the DT bindings documentation for the Sparx5 SoC DPLL clock

Link: https://lore.kernel.org/r/20200615133242.24911-7-lars.povlsen@microchip.comReviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>