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

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>