docs: Fix htmldocs build failure

docs: Fix htmldocs build failure

Build of HTML docs failing due to conversion of deviceiobook.tmpl in
2bc7d62b and regulator.tmpl in ebff9acf to RST without removing from
DOCBOOKS in Makefile, resulting (in the case of deviceiobook) the
following error:

make[1]: *** No rule to make target 'Documentation/DocBook/deviceiobook.xml', needed by 'Documentation/DocBook/deviceiobook.aux.xml'.  Stop.
Makefile:1452: recipe for target 'htmldocs' failed
make: *** [htmldocs] Error 2

Update DOCBOOKS to reflect available books.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>