irq_domain: add documentation and MAINTAINERS entry.

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

v4: editorial changes

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Reviewed-by: Randy 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: Olof Johansson <olof@lixom.net>