Documentation: Update IRQ-domain.txt to document irq_domain_mapping

irq_domain_mapping is a rather useful tool to understand how IRqs
are mapped in irqdomains, and yet it is not documented anywhere.
Let's address this.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: http://lkml.kernel.org/r/20170512115538.10767-5-marc.zyngier@arm.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>