mtd: rawnand: rockchip: Use flexible-array member instead of zero-length array

Suppresses the following coccinelle warning:

drivers/mtd/nand/raw/rockchip-nand-controller.c:162:4-8: WARNING use flexible-array member instead

Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210323131137.45552-1-zou_wei@huawei.com