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

Add initial support for the Renesas Salvator-X 2nd version development
board equipped with an R-Car M3-W+ SiP with 8 (2 x 4) GiB of RAM.

The memory map is as follows:
  - Bank0: 4GiB RAM : 0x000048000000 -> 0x000bfffffff
		      0x000480000000 -> 0x004ffffffff
  - Bank1: 4GiB RAM : 0x000600000000 -> 0x006ffffffff

Based on a patch in the BSP by Takeshi Kihara
<takeshi.kihara.df@renesas.com>.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20191023123342.13100-10-geert+renesas@glider.be