irq_domain: Remove references to old irq_host names

No functional changes.  Replaces non-exported references to 'host' with domain.
Does not change any symbol names referenced by other .c files.
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
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>