dt-bindings: Add vendor prefix for Adafruit

This adds a device tree vendor prefix for Adafruit Industries, LLC.

Signed-off-by: David Lechner <david@lechnology.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525193623.15533-3-david@lechnology.com