irqchip/renesas-irqc: Use linear IRQ domain

Use linear IRQ domain instead of irq_domain_add_simple() that also
handles non-DT cases. This reduces the delta between the IRQC code 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/20150720100625.2552.63939.sendpatchset@little-apple


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