ARM: shmobile: Document Renesas M3-W-based Salvator-XS board DT bindings

The Renesas Salvator-XS (Salvator-X 2nd version) development board can
be equipped with either an R-Car H3 ES2.0 or M3-W ES1.x SiP, which are
pin-compatible.

Document board part number and compatible values for the version with
R-Car M3-W.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

Documentation/devicetree/bindings/arm/shmobile.txt

@@ -104,6 +104,8 @@ Boards:
     compatible = "renesas,salvator-x", "renesas,r8a7796"
   - Salvator-XS (Salvator-X 2nd version, RTP0RC7795SIPB0012S)
     compatible = "renesas,salvator-xs", "renesas,r8a7795"
+  - Salvator-XS (Salvator-X 2nd version, RTP0RC7796SIPB0012S)
+    compatible = "renesas,salvator-xs", "renesas,r8a7796"
   - SILK (RTP0RC7794LCB00011S)
     compatible = "renesas,silk", "renesas,r8a7794"
   - SK-RZG1E (YR8A77450S000BE)