arm: omap2/3: Use generic irq chip

Use generic irq chip for omap2 & 3.

Note that this patch also leaves out the spurious IRQ warning
for omap3.

This warning should no longer be needed as the interrupt handlers
for various devices have implemented the necessayr read-back of
the posted write.

Signed-off-by: Tony Lindgren <tony@atomide.com>