ARM: mvebu: Enable NAND controller in Armada 370 Mirabox

The Armada 370 Mirabox has a NAND flash, so enable it in the devicetree
and add the partitions as prepared in the factory images.

In order to skip the driver's custom device detection and use only
ONFI detection, the "marvell,keep-config" parameter is used.
This is needed because we have no support for setting the timings parameters yet.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>