arm64: dts: m3ulcb: add M3ULCB board DT bindings

Add M3ULCB Device tree bindings Documentation, listing it as a supported
board.
Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

Documentation/devicetree/bindings/arm/shmobile.txt

@@ -61,6 +61,8 @@ Boards:
     compatible = "renesas,kzm9g", "renesas,sh73a0"
   - Lager (RTP0RC7790SEB00010S)
     compatible = "renesas,lager", "renesas,r8a7790"
+  - M3ULCB (R-Car Starter Kit Pro, RTP0RC7796SKB00010S)
+    compatible = "renesas,m3ulcb", "renesas,r8a7796";
   - Marzen
     compatible = "renesas,marzen", "renesas,r8a7779"
   - Porter (M2-LCDP)