dt-bindings: watchdog: aspeed,ast2400-wdt: Convert to DT schema

Squash warnings such as:

```
arch/arm/boot/dts/aspeed/aspeed-bmc-facebook-galaxy100.dtb: /ahb/apb@1e600000/watchdog@1e785000: failed to match any schema with compatible: ['aspeed,ast2400-wdt']
```

The schema binding additionally defines the clocks property over the
prose binding to align with use of the node in the DTS files.

Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20240403020439.418788-1-andrew@codeconstruct.com.au


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>