arm: omap: intc: switch over to linear irq domain

now that we don't need to support legacy board-files,
we can completely switch over to a linear irq domain
and make use of irq_alloc_domain_generic_chips() to
allocate all generic irq chips for us.

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