irqchip: intc-irqpin: Cache mapped IRQ

Cache IRQ in domain_irq variable instead of
making use of irq_find_mapping(). While at it
rename the irq variable to requested_irq.

Signed-off-by: Magnus Damm <damm@opensource.se>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>