dt-bindings: irq: sun7i-nmi: Add binding documentation for the V3s NMI

The V3s NMI controller seems register-compatible with the A80 (sun9i).
Add new items for the compatible string, with an entry specific to the V3s
and the A80 entry.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20201206165131.1041983-2-contact@paulk.fr