ARM: OMAP: Avoid cpu_is_omapxxxx usage until map_io is done

This way we don't need to initialize SoC detection early
and can start using generic map_io.

Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>