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/at91sam9x5ek_nandflash_defconfig b/configs/at91sam9x5ek_nandflash_defconfig
index d1a8c6c..004ee25 100644
--- a/configs/at91sam9x5ek_nandflash_defconfig
+++ b/configs/at91sam9x5ek_nandflash_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