ARM: shmobile: Add silk device tree bindings documentation

Add Silk Device tree bindings Documentation, listing it as a supported board.

This allows to use checkpatch to validate DTSes referring to the Silk board.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>

Documentation/devicetree/bindings/arm/shmobile.txt

@@ -55,3 +55,5 @@ Boards:
     compatible = "renesas,lager", "renesas,r8a7790"
   - Marzen
     compatible = "renesas,marzen", "renesas,r8a7779"
+  - Silk
+    compatible = "renesas,silk", "renesas,r8a7794";