[media] DocBook: fix various validation errors

Fixed the following errors (with exception of the SVG errors):
  GEN     /home/hans/work/src/v4l/media-git/Documentation/DocBook//v4l2.xml
  rm -rf Documentation/DocBook/index.html; echo '<h1>Linux Kernel HTML Documentation</h1>' >> Documentation/DocBook/index.html && echo '<h2>Kernel Version: 3.8.0-rc1</h2>' >> Documentation/DocBook/index.html && cat Documentation/DocBook/media_api.html >> Documentation/DocBook/index.html
  /tmp/x.xml:883: element revremark: validity error : Element structname is not declared in revremark list of possible children
  /tmp/x.xml:883: element revremark: validity error : Element xref is not declared in revremark list of possible children
  /tmp/x.xml:9580: element xref: validity error : Element xref was declared EMPTY this one has content
  /tmp/x.xml:13508: element link: validity error : Element link does not carry attribute linkend
  /tmp/x.xml:13508: element link: validity error : No declaration for attribute linked of element link
  /tmp/x.xml:16986: element imagedata: validity error : Value "SVG" for attribute format of imagedata is not among the enumerated set
  /tmp/x.xml:17003: element imagedata: validity error : Value "SVG" for attribute format of imagedata is not among the enumerated set
  /tmp/x.xml:17022: element imagedata: validity error : Value "SVG" for attribute format of imagedata is not among the enumerated set

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>