The clocks property is mandatory, document it.
Reported-by: NRob Herring <robh+dt@kernel.org>
Acked-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200128204200.22201-1-alexandre.belloni@bootlin.comSigned-off-by: NAlexandre Belloni <alexandre.belloni@bootlin.com>