irq_domain: Make irq_domain structure match powerpc's irq_host

Part of the series to unify the irq remapping mechanisms in the
kernel.  A follow up patch will copy the powerpc implementation into
kernel/irq/irqdomain.c, which will be a lot easier if the structures
are identical.

Where they differ, I've chose to use the powerpc names since there is
a lot more code using those names.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Milton Miller <miltonm@bga.com>
Tested-by: Olof Johansson <olof@lixom.net>