Add support to fdt_slicer for the new style partition data documented in
devicetree/bindings/mtd/partition.txt.

In the old style, all the children of the device node which did not have a
compatible property were the partitions. In the new style, there is a child
node of the device which has a compatible string of "fixed-partitions", and
its children are the individual partitions.

Also, support the read-only property by setting the corresponding slice flag.