ARM: shmobile: silk: 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. Hence move the SPI FLASH partitions to a
"partitions" subnode.

Based on similar work for the koelsch board by Geert Uytterhoeven.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>