dt-bindings: arm: fsl: Add ZII Ultra boards binding

Document the binding for Zodiac Inflight Innovations Ultra Boards.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>