OMAP3: igep00x0: Add config option to choose flash storage memory

IGEP-based boards can have two different flash memories, a OneNAND or a
NAND device. Add a configuration option for to choose which memory to use.

Signed-off-by: Javier Martinez Canillas <javier@dowhile0.org>
Acked-by: Enric Balletbo i Serra <eballetbo@gmail.com>