ARM: OMAP: Clean up interrupt lines to fix warnings for multi-omap

If boards with different NR_IRQS are compiled together, tons of
compiler warnings are emitted about redefining NR_IRQS.

This patch fixes the problem by adding up NR_IRQS in a common place.

Patch also removes quite a bit of now unnecessary code.

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