ARM: dt: mvebu: ix4-300d: move partitions to partition sub-node

NAND flash partitions should be part of a partitions sub-node
not the flash node itself. Move the partitions which will also
allow different bootloaders get rid of the stock partitions
easily by removing the partitions node.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>