media: admin-guide: Fix mgb4 driver documentation structure

Fix the mgb4 driver documentation structure that breaks the
"Video4Linux (V4L) driver-specific documentation" outline.

Signed-off-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

media: Documentation: Added Digiteq Automotive MGB4 driver documentation

The "admin-guide" documentation for the Digiteq Automotive MGB4 driver.

Signed-off-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>