ARM: shmobile: document iW-RainboW-G22D SODIMM SOM Development Platform

Document the iW-RainboW-G22D device tree bindings.
It is just a placeholder for the time being, the actual
implementation is not available yet.
Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Chris Paterson <chris.paterson2@renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

Documentation/devicetree/bindings/arm/shmobile.txt

@@ -68,6 +68,8 @@ Boards:
     compatible = "renesas,h3ulcb", "renesas,r8a7795";
   - Henninger
     compatible = "renesas,henninger", "renesas,r8a7791"
+  - iWave Systems RZ/G1E SODIMM SOM Development Platform (iW-RainboW-G22D)
+    compatible = "iwave,g22d", "iwave,g22m", "renesas,r8a7745"
   - iWave Systems RZ/G1E SODIMM System On Module (iW-RainboW-G22M-SM)
     compatible = "iwave,g22m", "renesas,r8a7745"
   - iWave Systems RZ/G1M Qseven Development Platform (iW-RainboW-G20D-Qseven)