mtd: spi-nor: explicitly include <linux/of.h>

The core driver is using of_property_read_bool() and relies on implicit
inclusion of <linux/of.h>, which comes from <linux/mtd/mtd.h>.

It is good practice to directly include all headers used, it avoids
implicit dependencies and spurious breakage if someone rearranges
headers and causes the implicit include to vanish.

Include the missing header.

Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20250307-spi-nor-headers-cleanup-v1-1-c186a9511c1e@linaro.org


Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>