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

Documentation/devicetree/bindings/arm/shmobile.txt

@@ -39,6 +39,8 @@ Boards:
     compatible = "renesas,ape6evm", "renesas,r8a73a4"
   - Atmark Techno Armadillo-800 EVA
     compatible = "renesas,armadillo800eva"
+  - Blanche (RTP0RC7792SEB00010S)
+    compatible = "renesas,blanche", "renesas,r8a7792"
   - BOCK-W
     compatible = "renesas,bockw", "renesas,r8a7778"
   - Genmai (RTK772100BC00000BR)