Documentation for irq_domain library which will be created in subsequent
patches.

v4: editorial changes
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>
Reviewed-by: NRandy Dunlap <rdunlap@xenotime.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Milton Miller <miltonm@bga.com>
Tested-by: NOlof Johansson <olof@lixom.net>