regulator: Add basic DocBook manual

Add a basic DocBook manual for the regulator API. This is much more
skeletal than the existing text documentation, the main benefit is to
provide a skeleton for automatic generation of a manual based on the
kerneldoc for the API.

Since large portions of the text are lifted from the existing text format
documentation written by Liam Girdwood much of the credit belongs to
him.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>

Documentation/DocBook/Makefile

@@ -12,7 +12,7 @@ DOCBOOKS := z8530book.xml mcabook.xml \
 	    kernel-api.xml filesystems.xml lsm.xml usb.xml kgdb.xml \
 	    gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml \
 	    genericirq.xml s390-drivers.xml uio-howto.xml scsi.xml \
-	    mac80211.xml debugobjects.xml sh.xml
+	    mac80211.xml debugobjects.xml sh.xml regulator.xml
 
 ###
 # The build process is as follows (targets):