ARM: omap2/3: Add global omap2/3_intc_handle_irq() functions

Provide the OMAP2/3 IRQ code with low level handlers that can be used
by platforms using CONFIG_MULTI_IRQ_HANDLER. Though the handlers are
written in C, the compiled code looks very similar to its assembly
counterpart (at least with my gcc 4.4.1).

Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>