ARM: at91/dt: binding: add precision to AIC documentation

In response to the "undocumented compatible strings" message, here is a
patch which is adding the precision of two "chips" that should be used for
the "atmel,<chip>-aic" compatibility string.
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>

Documentation/devicetree/bindings/arm/atmel-aic.txt

@@ -2,6 +2,7 @@
 
 Required properties:
 - compatible: Should be "atmel,<chip>-aic"
+  <chip> can be "at91rm9200" or "sama5d3"
 - interrupt-controller: Identifies the node as an interrupt controller.
 - interrupt-parent: For single AIC system, it is an empty property.
 - #interrupt-cells: The number of cells to define the interrupts. It should be 3.