genirq/irqdomain: Implement get_name() method on irqchip fwnodes

Prerequisite to make x86 more irqdomain compliant.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201024213535.443185-23-dwmw2@infradead.org