memory: atmel-ebi: Properly handle multiple reference to the same CS

Some devices are defining several sub-ranges within the same CS iomem
range. In this case, we should not duplicate the EBI device config.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>