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>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210917103055.92150-1-bigeasy@linutronix.de