UIO: add UIO documentation target to DocBook Makefile

Add the DocBook documentation for the Userspace I/O framework
to the Makefile.

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: Hans J Koch <hjk@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>