davinci: omapl138-lcdk: enable driver model for NAND

Enable the driver-model on da850-lcdk. We need to add a dummy nand node
to the device tree, as the real nand node is a sub-node of the aemif
device.

On linux the aemif driver populates all its child nodes, but we can't do
it in u-boot currently.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>