dt-bindings: arm: Document Renesas R-Car M3-N-based Salvator-X board

The Renesas Salvator-X development board can be equipped with an R-Car
H3, M3-W, or M3-N SiP, which are pin-compatible.

Document board part number and compatible values for the version with
R-Car M3-N.

The board part number was extracted from a big patch by Takeshi Kihara
in the BSP.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

Documentation/devicetree/bindings/arm/shmobile.txt

@@ -108,6 +108,8 @@ Boards:
     compatible = "renesas,salvator-x", "renesas,r8a7795"
   - Salvator-X (RTP0RC7796SIPB0011S)
     compatible = "renesas,salvator-x", "renesas,r8a7796"
+  - Salvator-X (RTP0RC7796SIPB0011S (M3N))
+    compatible = "renesas,salvator-x", "renesas,r8a77965"
   - Salvator-XS (Salvator-X 2nd version, RTP0RC7795SIPB0012S)
     compatible = "renesas,salvator-xs", "renesas,r8a7795"
   - Salvator-XS (Salvator-X 2nd version, RTP0RC7796SIPB0012S)