Documentation: update the devices.txt documentation

Alan is no longer maintaining this list through the Linux assigned
numbers authority. Make it a collective document by referring to
"the maintainers" in plural throughout, and naming the chardev and
block layer maintainers in particular as parties of involvement.
Cut down and remove some sections that pertained to the process of
maintaining the list at lanana.org and contacting Alan directly.

Make it clear that this document, in the kernel, is the master
document.

Also move paragraphs around so as to emphasize dynamic major number
allocation.

Remove paragraph on 2.6 deprecation, that tag no longer appears
anywhere in the file.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>