irqchip/sunxi-nmi: Add sun9i-a80 variant to binding doc

sun9i A80 introduces a new variant of the NMI controller. The registers
are reordered, but the functionality remains the same.

Add a new compatible string for it.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lkml.kernel.org/r/1449130813-22400-3-git-send-email-wens@csie.org
Signed-off-by: Jason Cooper <jason@lakedaemon.net>