genirq/irqdomain: Allow partial trimming of irq_data hierarchy

It appears that some HW is ugly enough that not all the interrupts
connected to a particular interrupt controller end up with the same
hierarchy depth (some of them are terminated early). This leaves
the irqchip hacker with only two choices, both equally bad:

- create discrete domain chains, one for each "hierarchy depth",
  which is very hard to maintain

- create fake hierarchy levels for the shallow paths, leading
  to all kind of problems (what are the safe hwirq values for these
  fake levels?)

Implement the ability to cut short a single interrupt hierarchy
from a level marked as being disconnected by using the new
irq_domain_disconnect_hierarchy() helper.

The irqdomain allocation code will then perform the trimming

Signed-off-by: Marc Zyngier <maz@kernel.org>

include/linux/irqdomain.h

@@ -509,6 +509,9 @@ extern void irq_domain_free_irqs_parent(struct irq_domain *domain,
 					unsigned int irq_base,
 					unsigned int nr_irqs);
 
+extern int irq_domain_disconnect_hierarchy(struct irq_domain *domain,
+					   unsigned int virq);
+
 static inline bool irq_domain_is_hierarchy(struct irq_domain *domain)
 {
 	return domain->flags & IRQ_DOMAIN_FLAG_HIERARCHY;