ARM: mvebu: Enable NAND controller in A370 Reference Design board

Marvell's Armada 370 Reference Design 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>