irqchip/renesas-irqc: Move over to nested generic chip

Convert the IRQC driver to rely on GENERIC_IRQ_CHIP and
set IRQ_GC_INIT_NESTED_LOCK to enable nested locking.

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/20150928094237.32552.83434.sendpatchset@little-apple


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