This hooks in irq_to_desc_alloc_cpu() to the necessary code paths in the
intc and ipr controller registration paths. As these are the primary call
paths for all SH CPUs, this alone will make all CPUs sparse IRQ ready.

There is the added benefit now that each CPU contains specific IPR and
INTC tables, so only the vectors with interrupt sources backing them will
ever see an irq_desc instantiation. This effectively packs irq_desc
down to match the CPU, rather than padding NR_IRQS out to cover the valid
vector range.

Boards with extra sources will still have to fiddle with the nr_irqs
setting, but they can continue doing so through the machvec as before.
Signed-off-by: Paul Mundt <lethal@linux-sh.org>