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>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>