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/T4160QDS_NAND_defconfig b/configs/T4160QDS_NAND_defconfig
index d98af62..bf88846 100644
--- a/configs/T4160QDS_NAND_defconfig
+++ b/configs/T4160QDS_NAND_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="PPC_T4160,RAMBOOT_PBL,SPL_FSL_PBL,NAND"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y