dt-bindings: arm: renesas: Add Salvator-XS board with R-Car M3-W+

Add device tree binding documentation for the Renesas Salvator-XS board
equipped with an R-Car M3-W+ (R8A77961) SoC.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Link: https://lore.kernel.org/r/20191023122911.12166-3-geert+renesas@glider.be

Documentation/devicetree/bindings/arm/renesas.yaml

@@ -207,6 +207,8 @@ properties:
 
       - description: R-Car M3-W+ (R8A77961)
         items:
+          - enum:
+              - renesas,salvator-xs # Salvator-XS (Salvator-X 2nd version, RTP0RC7796SIPB0012SA5A)
           - const: renesas,r8a77961
 
       - description: Kingfisher (SBEV-RCAR-KF-M03)