Proposal about reorganize struct irq_data and struct irq_desc

[Jiang Liu <jiang.liu@linux.intel.com>]

Hi Thomas and Marc,
	During working on the generic MSI support, I have some proposal
about reorganizing struct irq_data and struct irq_desc. The proposed
changes are:
1) Add a pointer "struct irq_desc *" to struct irq_data, so we could
   quickly get struct irq_desc from struct irq_data.
2) Move "node" from struct irq_data into struct irq_desc, NUMA info
   should be per-irq instead of per-chip.
3) Move "affinity" from struct irq_data into struct irq_desc, NUMA info
   should be per-irq instead of per-chip.
4) Move "msi_desc" from struct irq_data into struct irq_desc. (Not sure
   whether we should do this. Theoretically we should use
   irq_data->handler_data to store msi_desc.)

With above change applied, struct irq_data only hosts per-chip data, and
struct irq_desc hosts per-irq data. What's your thoughts?
Regards,
Gerry