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/ethernut5_defconfig b/configs/ethernut5_defconfig
index 2645d90..04d8611 100644
--- a/configs/ethernut5_defconfig
+++ b/configs/ethernut5_defconfig
@@ -7,3 +7,4 @@
 # CONFIG_CMD_LOADS is not set
 # CONFIG_CMD_FPGA is not set
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_ATMEL=y