omap3: introduce omap3_map_io

Most OMAP3-based boards use exactly the same code for .map_io method in
the machine_desc structure.
This patch introduces omap3_map_io and updates board-* files to use it
as .map_io method.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>