arm64: renesas: document V3MSK board bindings

Document the V3M Starter Kit device tree bindings, listing it as
a supported board.

This allows to use checkpatch.pl to validate .dts files referring to
the V3MSK board.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>