The dtschema version check works, but is not that clear when dtschema is
either not installed or not in the PATH. Add a separate check and
message if dt-doc-validate is not found.

Cc: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>