arm64: dts: renesas: rcar-gen3: Add SoC model to comment headers

Make sure the R-Car Gen3 SoC model present is documented in the comment
header of each board DTS, on a single line.  This makes it easier to
identify boards that are available with different SoC or SiP options.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/251569665d7d4f4ed4bbab7267ce2ddccdef33e5.1626261816.git.geert+renesas@glider.be