irqchip: renesas-irqc: Use lazy disable

Set the ->irq_enable() and ->irq_disable() methods to NULL
to enable lazy disable of interrupts. This by itself provides
some level of optimization, but is mainly enabled as ground
work for future Suspend-to-RAM wake up support.

Signed-off-by: Magnus Damm <damm@opensource.se>
Cc: rob.herring@calxeda.com
Cc: grant.likely@secretlab.ca
Cc: horms@verge.net.au
Link: http://lkml.kernel.org/r/20131204120546.29642.15772.sendpatchset@w520
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>