mtd: spi-nor: Expose stuctures and functions to manufacturer drivers

Expose the flash_info struct and some function prototypes that
will be used by manufacturers.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>