Document the SK-RZG1M device tree bindings, listing it as a supported board.

This allows to use checkpatch.pl to validate .dts files referring to the
SK-RZG1M board.
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>