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