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 <[e-mail]>
Acked-by: Maxime Ripard <[e-mail]>
Reviewed-by: Hans de Goede <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Jason Cooper <[e-mail]>

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

···
 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