dt-bindings: timer: cadence_ttc: Add power-domains

Describe optional power-domain property to fix dts_check warnings.
The similar change was done by commit 8c0aa567 ("dt-bindings: gpio:
fsl-imx-gpio: Add power-domains").
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/cc655a72b20790f6d7408b1aaf81c4bf878aafb4.1634286552.git.michal.simek@xilinx.comSigned-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/timer/cdns,ttc.yaml

@@ -25,6 +25,9 @@ properties:
   clocks:
     maxItems: 1
 
+  power-domains:
+    maxItems: 1
+
   timer-width:
     $ref: "/schemas/types.yaml#/definitions/uint32"
     description: |