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>