memory: omap-gpmc: Prevent mapping into 1st 16MB

We have been preventing mapping GPMC children in the
first 1MB but really it has to be the first 16MB as
the minimum GPMC partition size is 16MB.

Also print an error message if CS mapping fails
due to DT requesting address outside the GPMC
map.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>