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/marsboard_defconfig b/configs/marsboard_defconfig
index a353e85..8495800 100644
--- a/configs/marsboard_defconfig
+++ b/configs/marsboard_defconfig
@@ -7,4 +7,5 @@
 CONFIG_CMD_GPIO=y
 CONFIG_DM=y
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y
 CONFIG_DM_THERMAL=y