mtd: spi-nor: include mtd.h header for struct mtd_info definition

So far struct spi_nor was using just a pointer to struct mtd_info so it
wasn't needed to have it fully defined there. After recent change we
embed whole struct so we need to include a proper header.

Fixes: 19763671 ("mtd: spi-nor: embed struct mtd_info within struct spi_nor")
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>