arm64: dts: renesas: Add support for Salvator-XS with R-Car M3-W

Add initial support for the Renesas Salvator-XS (Salvator-X 2nd version)
development board equipped with an R-Car M3-W SiP.

Based on work for the Salvator-X and -XS boards with M3-W resp. H3 SiPs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>