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>