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/T4240QDS_SECURE_BOOT_defconfig b/configs/T4240QDS_SECURE_BOOT_defconfig
index f0ce69a..861ac5c 100644
--- a/configs/T4240QDS_SECURE_BOOT_defconfig
+++ b/configs/T4240QDS_SECURE_BOOT_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_T4240QDS=y
 CONFIG_SYS_EXTRA_OPTIONS="PPC_T4240,SECURE_BOOT"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y