Please make sure the followed setting had been included in Uboot config file "mex28_evk.h":
/*
* SPI Driver
*/
#define CONFIG_CMD_SPI
#define CONFIG_CMD_SF
#ifdef CONFIG_CMD_SPI
#define CONFIG_MXS_SPI
#define CONFIG_DEFAULT_SPI_BUS 2
#define CONFIG_DEFAULT_SPI_MODE SPI_MODE_3
#define CONFIG_SF_DEFAULT_SPEED 20000000
#define CONFIG_SPI_FLASH
#define CONFIG_SPI_FLASH_SPANSION
#define CONFIG_SPI_FLASH_CS 0
#endif