dt-bindings: arm: Document Renesas H2-based Stout DT bindings

Document the Renesas H2 Stout (ADAS Starterkit) device tree bindings,
listing it as a supported board.

This allows to use checkpatch.pl to validate .dts files referring to
the Stout board.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>