dt-bindings: arm: renesas: Document iW-RainboW-G21M-Qseven-RZG1H SoM

Document the iW-RainboW-G21M-Qseven-RZG1H device tree bindings,
listing it as a supported system on module.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Marian-Cristian Rotariu <marian-cristian.rotariu.rb@bp.renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1588542414-14826-8-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com


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