mtd: ofpart: add compatible check for child nodes

In case that the nand device will support some features like Nand Flash
Controller, we want to make the sub feature as a sub node of nand device.

Use such organization it is easy to enable/disable feature, also it is back
compatible and more readable.

If the sub-node has a compatible property then it is a driver not partition.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
[ added a missing newline -Brian ]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>