docbook: split kernel-api for device-drivers

The kernel-api docbook was much larger than any of the others,
so processing it took longer and needed some docbook extras in
some cases, so split it into kernel-api (infrastructure etc.)
and device drivers/device subsystems.  This allows these docbooks
to be generated in parallel.  (This reduced the docbook processing
time on my 4-proc system with make -j4 from about 5min:16sec to
about 2min:01sec.)

The chapters that were moved from kernel-api to device-drivers are:

Driver Basics
Device drivers infrastructure
Parallel Port Devices
Message-based devices
Sound Devices
16x50 UART Driver
Frame Buffer Library
Input Subsystem
Serial Peripheral Interface (SPI)
I2C and SMBus Subsystem
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/DocBook/Makefile

@@ -6,7 +6,7 @@
 # To add a new book the only step required is to add the book to the
 # list of DOCBOOKS.
 
-DOCBOOKS := z8530book.xml mcabook.xml \
+DOCBOOKS := z8530book.xml mcabook.xml device-drivers.xml \
 	    kernel-hacking.xml kernel-locking.xml deviceiobook.xml \
 	    procfs-guide.xml writing_usb_driver.xml networking.xml \
 	    kernel-api.xml filesystems.xml lsm.xml usb.xml kgdb.xml \