Generialize powerpc's irq_host as irq_domain

This branch takes the PowerPC irq_host infrastructure (reverse mapping
from Linux IRQ numbers to hardware irq numbering), generalizes it,
renames it to irq_domain, and makes it available to all architectures.

Originally the plan has been to create an all-new irq_domain
implementation which addresses some of the powerpc shortcomings such
as not handling 1:1 mappings well, but doing that proved to be far
more difficult and invasive than generalizing the working code and
refactoring it in-place.  So, this branch rips out the 'new'
irq_domain and replaces it with the modified powerpc version (in a
fully bisectable way of course).  It converts all users over to the
new API and makes irq_domain selectable on any architecture.

No architecture is forced to enable irq_domain, but the infrastructure
is required for doing OpenFirmware style irq translations.  It will
even work on SPARC even though SPARC has it's own mechanism for
translating irqs at boot time.  MIPS, microblaze, embedded x86 and c6x
are converted too.

The resulting irq_domain code is probably still too verbose and can be
optimized more, but that can be done incrementally and is a task for
follow-on patches.
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