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/nitrogen6q2g_defconfig b/configs/nitrogen6q2g_defconfig
index 9938328..e5c7824 100644
--- a/configs/nitrogen6q2g_defconfig
+++ b/configs/nitrogen6q2g_defconfig
@@ -6,3 +6,4 @@
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y