irqchip/renesas-irqc: Make use of irq_find_mapping()

Instead of locally caching the virq as domain_irq simply rely on the
IRQ domain code and irq_find_mapping().  This reduces the delta
between the IRQC driver and the generic chip implementation.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Cc: jason@lakedaemon.net
Cc: geert+renesas@glider.be
Cc: horms@verge.net.au
Cc: Magnus Damm <magnus.damm@gmail.com>
Link: http://lkml.kernel.org/r/20150720100635.2552.20906.sendpatchset@little-apple


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