ARM: dts: lpc32xx: phy3250: add NAND partitions device node

To declare MTD OF partitions NAND controller device node should have
a special 'partitions' subnode, the change removes a debug message
from mtd/ofpart on boot:

  nxp_lpc3220_slc: 'partitions' subnode not found on /ahb/flash@20020000.
  Trying to parse direct subnodes as partitions.

Acked-by: Sylvain Lemieux <slemieux.tyco@gmail.com>
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>