sf: Move SPI flash drivers to defconfig

There are already Kconfig options for SPI flash drivers, but we
have not moved them from config.h to defconfig files. This commit
does this in a batch.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/configs/mx28evk_nand_defconfig b/configs/mx28evk_nand_defconfig
index bd86917..f4ace44 100644
--- a/configs/mx28evk_nand_defconfig
+++ b/configs/mx28evk_nand_defconfig
@@ -6,3 +6,4 @@
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y