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

Most features are enabled through the shared salvator-xs.dtsi board
description.  The memory configuration is specific to the M3-N SiP.
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
[geert: Switch to SPDX-License-Identifier, update patch description]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>