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/atngw100mkii_defconfig b/configs/atngw100mkii_defconfig
index 48ea9ba..b552421 100644
--- a/configs/atngw100mkii_defconfig
+++ b/configs/atngw100mkii_defconfig
@@ -10,3 +10,4 @@
 # CONFIG_CMD_FPGA is not set
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_ATMEL=y