dt-bindings: gpio: aspeed,ast2400-gpio: Convert to DT schema

Squash warnings such as:

```
arch/arm/boot/dts/aspeed/aspeed-bmc-facebook-galaxy100.dtb: /ahb/apb@1e600000/gpio@1e780000: failed to match any schema with compatible: ['aspeed,ast2400-gpio']
```

Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>