The bootloader adds MTD partitions in this subnode if present, or in the
spi-nor node itself otherwise. Direct children cause warnings like:
425e0000.spi: ofpart partition /soc@0/bus@42000000/spi@425e0000/flash@0/
  partition@600000 (/soc@0/bus@42000000/spi@425e0000/flash@0) #size-cells
  is wrongly set to <0>, assuming <1> for parsing partitions.

This is due flexspi node having #size-cells = <0>. Setting #size-cells in
MTD nodes itself is deprecated by mtd.yaml.
Fix all this by adding an empty partitions node which the bootloader will
fill with configured MTD partitions.
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>