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 <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Milton Miller <[email protected]>
Tested-by: Olof Johansson <[email protected]>