ARM: dts: document Wheat board

Document the Wheat device tree bindings, listing it as a supported board.

This allows to use checkpatch.pl to validate .dts files referring to the
Wheat board.
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

Documentation/devicetree/bindings/arm/shmobile.txt

@@ -69,3 +69,5 @@ Boards:
     compatible = "renesas,salvator-x", "renesas,r8a7796";
   - SILK (RTP0RC7794LCB00011S)
     compatible = "renesas,silk", "renesas,r8a7794"
+  - Wheat
+    compatible = "renesas,wheat", "renesas,r8a7792"