memory: omap-gpmc: Make 'bank-width' property optional

Error out only if both 'bank-width' and 'gpmc,device-width' are missing.
As 'bank-width' is mostly used for NOR devices and all other devices must
use 'gpmc,device-width' update the error message accordingly.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Roger Quadros <rogerq@ti.com>