genirq: irqchip: Add mask to block out invalid irqs

Some controllers have irqs that aren't wired up and must never be used.
For the generic chip attached to an irq_domain this provides a mask that
can be used to block out particular irqs so that they never get mapped.

Signed-off-by: Grant Likely <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Thomas Gleixner <[e-mail]>

include/linux/irq.h

···
  * @num_ct:		Number of available irq_chip_type instances (usually 1)
  * @private:		Private data for non generic chip callbacks
  * @installed:		bitfield to denote installed interrupts
+ * @unused:		bitfield to denote unused interrupts
  * @domain:		irq domain pointer
  * @list:		List head for keeping track of instances
  * @chip_types:		Array of interrupt irq_chip_types
···
 	unsigned int		num_ct;
 	void			*private;
 	unsigned long		installed;
+	unsigned long		unused;
 	struct irq_domain	*domain;
 	struct list_head	list;
 	struct irq_chip_type	chip_types[0];

kernel/irq/generic-chip.c

···
 
 	idx = hw_irq % dgc->irqs_per_chip;
 
+	if (test_bit(idx, &gc->unused))
+		return -ENOTSUPP;
+
 	if (test_bit(idx, &gc->installed))
 		return -EBUSY;