dt-bindings: spi: spi-cadence: Describe power-domains property

ZynqMP Cadence SPI IP core has own power domain that's why describe it as
optional property.

Signed-off-by: Michal Simek <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/9874[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/spi-cadence.yaml

···
     enum: [ 0, 1 ]
     default: 0
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg