genirq: Document IRQCHIP_ONESHOT_SAFE flag

Add missing documentation of the flag.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/irq.h

@@ -355,6 +355,7 @@ struct irq_chip {
  * IRQCHIP_ONOFFLINE_ENABLED:	Only call irq_on/off_line callbacks
  *				when irq enabled
  * IRQCHIP_SKIP_SET_WAKE:	Skip chip.irq_set_wake(), for this irq chip
+ * IRQCHIP_ONESHOT_SAFE:	One shot does not require mask/unmask
  */
 enum {
 	IRQCHIP_SET_TYPE_MASKED		= (1 <<  0),