mtd: spi-nor: drop unused <linux/of_platform.h>

There's nothing used in the SPI NOR core from <linux/of_platform.h>,
drop the header inclusion.

Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20250307-spi-nor-headers-cleanup-v1-3-c186a9511c1e@linaro.org


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