media: imx: set a media_device bus_info string

Some tools like v4l2-compliance let users select a media device based
on the bus_info string which can be quite convenient. Use a unique
string for that.

This also fixes the following v4l2-compliance warning:
warn: v4l2-test-media.cpp(52): empty bus_info

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>