dt-bindings: mtd: partition: Add missing type for "linux,rootfs"

"linux,rootfs" is missing a type, add it.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230613201014.2823185-1-robh@kernel.org