dt-bindings: irq: sun7i-nmi: Add binding for A100's NMI controller

Add a binding for A100's nmi controller.

Signed-off-by: Yangtao Li <frank@allwinnertech.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/953b76413563551b82dd11cadbc99c695f74f721.1595572867.git.frank@allwinnertech.com