arm: omap: irq: move some more code around

We want .init_irq to call set_irq_handle() for
legacy platforms. Note that this code will also
be dropped once omap2/3 devices are completely
moved to DT.

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