omap: Use separate init_irq functions to avoid cpu_is_omap tests early

This allows us to remove cpu_is_omap calls from init_irq functions.
There should not be any need for cpu_is_omap calls as at this point.
During the timer init we only care about SoC generation, and not about
subrevisions.

The main reason for the patch is that we want to initialize only
minimal omap specific code from the init_early call.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>