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_defconfig b/configs/T4160QDS_defconfig
index 2510208..d87eb23 100644
--- a/configs/T4160QDS_defconfig
+++ b/configs/T4160QDS_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_T4240QDS=y
 CONFIG_SYS_EXTRA_OPTIONS="PPC_T4160"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y