ARM: omap2plus: convert to CONFIG_MULTI_IRQ_HANDLER

Convert the omap2plus platforms to be using CONFIG_MULTI_IRQ_HANDLER.
Each machine is modified to provide either omap2_intc_handle_irq(),
omap3_intc_handle_irq() or gic_handle_irq().

This allows for a major cleanup, removing the MULTI_OMAP setup
from the interrupt path.

Tested on both Panda and IGEPv2 (single kernel image)

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