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/M54418TWR_nand_mii_defconfig b/configs/M54418TWR_nand_mii_defconfig
index 949be9a..373845b 100644
--- a/configs/M54418TWR_nand_mii_defconfig
+++ b/configs/M54418TWR_nand_mii_defconfig
@@ -8,3 +8,4 @@
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_ATMEL=y