dt-bindings: gpio: gpio-mvebu: convert txt binding to DT schema format

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: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>