arm64: dts: renesas: Add common Salvator-XS board support

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.

Add initial support for the common parts of the Salvator-XS board into
its own .dtsi file, to be included by the DTSes for the H3/M3-W
versions.

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