iio: distance: add dt binding for devantech-srf04

This patch adds dt binding for devantech ultrasonic ranger srf04.

The vendor "devantech" was already added to the vendor list with
        "[PATCH v4 1/3] iio: distance: srf08: add trivial DT binding"

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>