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_defconfig b/configs/T4240QDS_defconfig
index 2b9b145..7d8e826 100644
--- a/configs/T4240QDS_defconfig
+++ b/configs/T4240QDS_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_T4240QDS=y
 CONFIG_SYS_EXTRA_OPTIONS="PPC_T4240"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y