genirq: update irq_set_irqchip_state documentation

On -rt kernels, the use of migrate_disable()/migrate_enable() is
sufficient to guarantee a task isn't moved to another CPU.  Update the
irq_set_irqchip_state() documentation to reflect this.

Signed-off-by: Josh Cartwright <joshc@ni.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>