Preliminary irq registration call + irq capability checking

Need to add irqctrl capabilities and irq bits to device memory
caps.

Also need to initialize irq field of devmem caps.

include/l4/generic/container.h

@@ -60,7 +60,8 @@ struct cap_info {
 	unsigned long start;
 	unsigned long end;
 	unsigned long size;
-	unsigned int uattr[2];	/* User-level attributes (like device types) */
+	unsigned int attr;	/* Attributes (like device types) */
+	l4id_t irq;
 };