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/B4860QDS_SECURE_BOOT_defconfig b/configs/B4860QDS_SECURE_BOOT_defconfig
index c6a7cd7..1a61dc3 100644
--- a/configs/B4860QDS_SECURE_BOOT_defconfig
+++ b/configs/B4860QDS_SECURE_BOOT_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_B4860QDS=y
 CONFIG_SYS_EXTRA_OPTIONS="PPC_B4860,SECURE_BOOT"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y