ARM: OMAP: BUG if omap_ioremap is called before SoC detection

We don't have cpu_is_omapxxxx SoC detection initialized until
SoC detection is initialized from init_early. Just BUG for
now if omap_ioremap is being called too early.

Note that with the common map_io we should no longer need
cpu_is_omapxxxx for map_io.

Signed-off-by: Tony Lindgren <tony@atomide.com>