ARM: shmobile: document iW-RainboW-G20D-Qseven-RZG1M board

Document the iW-RainboW-G20D-Qseven-RZG1M device tree bindings,
listing it as a supported board.

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>