dt-bindings: arm: renesas: Document Renesas Spider boards

Add device tree bindings documentation for Renesas R-Car S4-8
Spider CPU and BreakOut boards.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20211201073308.1003945-2-yoshihiro.shimoda.uh@renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>