arm: omap: irq: switch over to intc_readl on omap_intc_handle_irq

an almost blind conversion from readl_relaxed
to our newly introduced intc_readl().

While at that, also remove some hardcoded
register addresses.

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