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.orgSigned-off-by: Jason Cooper <jason@lakedaemon.net>

Documentation/devicetree/bindings/interrupt-controller/allwinner,sunxi-nmi.txt

@@ -4,7 +4,7 @@ Allwinner Sunxi NMI Controller
 Required properties:
 
 - compatible : should be "allwinner,sun7i-a20-sc-nmi" or
-  "allwinner,sun6i-a31-sc-nmi"
+  "allwinner,sun6i-a31-sc-nmi" or "allwinner,sun9i-a80-nmi"
 - reg : Specifies base physical address and size of the registers.
 - interrupt-controller : Identifies the node as an interrupt controller
 - #interrupt-cells : Specifies the number of cells needed to encode an