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_SECURE_BOOT_defconfig b/configs/T4160QDS_SECURE_BOOT_defconfig
index ec5bef3..42e6508 100644
--- a/configs/T4160QDS_SECURE_BOOT_defconfig
+++ b/configs/T4160QDS_SECURE_BOOT_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_T4240QDS=y
 CONFIG_SYS_EXTRA_OPTIONS="PPC_T4160,SECURE_BOOT"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y