dt-bindings: irq: Convert Allwinner NMI Controller to a schema

The Allwinner SoCs have an interrupt controller called NMI supported in
Linux, with a matching Device Tree binding.

Now that we have the DT validation in place, let's convert the device tree
bindings for that controller over to a YAML schemas.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>