Allow fixed-partitions to be specified through a partitions
node.

Fixes the below dtbs_check warning:

arch/arm64/boot/dts/ti/k3-am642-evm-nand.dtb: nand@0,0: Unevaluated properties are not allowed ('partitions' was unexpected)
Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240903-gpmc-dtb-v2-1-8046c1915b96@kernel.org