dt-bindings: arm: renesas: Document iWave RZ/G1N SOM

Document the iW-RainboW-G20M-RZ/G1N Qseven device tree bindings,
listing it as a supported system on module.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>