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>