ARM: shmobile: add Porter board DT bindings

Add Porter device tree bindings documentation, listing it as a supported board.

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

Documentation/devicetree/bindings/arm/shmobile.txt

@@ -55,5 +55,7 @@ Boards:
     compatible = "renesas,lager", "renesas,r8a7790"
   - Marzen
     compatible = "renesas,marzen", "renesas,r8a7779"
+  - Porter (M2-LCDP)
+    compatible = "renesas,porter", "renesas,r8a7791"
   - Silk
     compatible = "renesas,silk", "renesas,r8a7794";