dt-bindings: arm: realtek: Document RTD1296 and Synology DS418

Define compatible strings for Realtek RTD1296 SoC and Synology
DiskStation DS418 NAS.

Cc: info@synology.com
Acked-by: Rob Herring <robh@kernel.org>
[AF: Converted to json-schema]
Signed-off-by: Andreas Färber <afaerber@suse.de>

Documentation/devicetree/bindings/arm/realtek.yaml

@@ -27,4 +27,10 @@ properties:
               - probox2,ava # ProBox2 AVA
               - zidoo,x9s # Zidoo X9S
           - const: realtek,rtd1295
+
+      # RTD1296 SoC based boards
+      - items:
+          - enum:
+              - synology,ds418 # Synology DiskStation DS418
+          - const: realtek,rtd1296
 ...