ARM: shmobile: koelsch: Move SPI FLASH partitions to subnode

As of commits 5cfdedb7 ("mtd: ofpart: move ofpart partitions to a
dedicated dt node") and fe2585e9

 ("doc: dt: mtd: support
partitions in a special 'partitions' subnode"), having partitions as
direct subnodes of an mtd device is discouraged:

    spi0.0: 'partitions' subnode not found on /spi@e6b10000/flash@0. Trying to parse direct subnodes as partitions.

Hence move the SPI FLASH partitions to a "partitions" subnode.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>