mtd: spi-nor: explicitly include <linux/math64.h>

swp and otp drivers use div_u64 and div64_u64 and rely on implicit
inclusion of <linux/math64.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: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/r/20250223-snor-math64-v2-1-6f0313eea331@linaro.org


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