ARM: mvebu: Enable NAND controller in Armada XP GP board

The Armada XP GP board has a NAND flash, so enable it in the devicetree.

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 haven't support for setting the timings
parameters yet and must rely in bootloader's.

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