fold struct irq_cfg into struct irq_desc

fold struct irq_cfg into struct irq_desc

struct irq_cfg really has become an architecture extension to struct
irq_desc, and hence it should be treated as such (rather than as IRQ
chip specific data, which it was meant to be originally).

For a first step, only convert a subset of the uses; subsequent
patches (partly to be sent later) will aim at fully eliminating the
use of the old structure type.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>