ARM: dts: blanche: document Blanche board

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

This allows to use checkpatch.pl to validate .dts files referring to the
Blanche board.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>