FireWire: add a Documentation driver-api chapter

Add a basic Firewire/IEEE 1394 driver API chapter to the Linux
kernel documentation.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Stefan Richter <stefanr@s5r6.in-berlin.de>
Cc: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Cc: linux-doc@vger.kernel.org
Cc: linux-scsi@vger.kernel.org
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "James E.J. Bottomley" <jejb@linux.vnet.ibm.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>