Documentation/: update 00-INDEX files

Some of the 00-INDEX files are somewhat outdated and some folders does
not contain 00-INDEX at all.  Only outdated (with the notably exception
of spi) indexes are touched here, the 169 folders without 00-INDEX has
not been touched.

New 00-INDEX
 - spi/* was added in a series of commits dating back to 2006

Added files (missing in (*/)00-INDEX)
 - dmatest.txt was added by commit 851b7e16 ("dmatest: run test via
   debugfs")
 - this_cpu_ops.txt was added by commit a1b2a555 ("percpu: add
   documentation on this_cpu operations")
 - ww-mutex-design.txt was added by commit 040a0a37 ("mutex: Add
   support for wound/wait style locks")
 - bcache.txt was added by commit cafe5635 ("bcache: A block layer
   cache")
 - kernel-per-CPU-kthreads.txt was added by commit 49717cb4
   ("kthread: Document ways of reducing OS jitter due to per-CPU
   kthreads")
 - phy.txt was added by commit ff764963 ("drivers: phy: add generic
   PHY framework")
 - block/null_bl...