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/atngw100_defconfig b/configs/atngw100_defconfig
index 5b01c18..7ef4677 100644
--- a/configs/atngw100_defconfig
+++ b/configs/atngw100_defconfig
@@ -11,3 +11,4 @@
 # CONFIG_CMD_SOURCE is not set
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_ATMEL=y