mtd: spi-nor: Introduce spi_nor_get_flash_info()

Dedicate a function for getting the pointer to the flash_info
const struct. Trim a bit the spi_nor_scan() huge function.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>