Convert the existing device tree binding to DT schema format.

The old binding listed the interrupt-controller and related properties
as required but there are sufficiently many existing usages without it
that the YAML binding does not make the interrupt properties required.
Signed-off-by: NChris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Reviewed-by: NKrzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: NBartosz Golaszewski <brgl@bgdev.pl>