dt-bindings: rtc: at91rm9200: document clocks property

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

Documentation/devicetree/bindings/rtc/atmel,at91rm9200-rtc.yaml

@@ -27,10 +27,14 @@ properties:
   interrupts:
     maxItems: 1
 
+  clocks:
+    maxItems: 1
+
 required:
   - compatible
   - reg
   - interrupts
+  - clocks
 
 additionalProperties: false