arm: omap: irq: call set_handle_irq() from .init_irq

the idea is that board-files won't need to set
.handle_irq on their machine_descs, which lets
us drop a little more pointless code.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>