ARM: mvebu: kirkwood: Add compatible property to "partitions" node

As of commit e488ca9f ("doc: dt: mtd: partitions: add compatible
property to "partitions" node"), the "partitions" subnode of an SPI
FLASH device node must have a compatible property. The partitions are no
longer detected if it is not present.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>