dt-bindings: arm: renesas: Document R-Car V3U SoC DT bindings

Add device tree bindings documentation for Renesas R-Car V3U
(r8a779a0).

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/1599470390-29719-2-git-send-email-yoshihiro.shimoda.uh@renesas.com


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