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 <michal.simek@amd.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/987430ee905fd299fe962663d94f848b341c87df.1691047461.git.michal.simek@amd.comSigned-off-by: Mark Brown <broonie@kernel.org>

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

@@ -49,6 +49,9 @@ properties:
     enum: [ 0, 1 ]
     default: 0
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg