arm64: dts: r8a7796: Add placeholders for various devices

Add empty device nodes serving as placeholders for devices that are not
yet supported and/or tested on R-Car M3-W, but are supported and used on
Salvator-X or H3ULCB boards equipped with an R-Car H3 SoC.

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