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/P5020DS_NAND_SECURE_BOOT_defconfig b/configs/P5020DS_NAND_SECURE_BOOT_defconfig
index b34f56b..46b78bc 100644
--- a/configs/P5020DS_NAND_SECURE_BOOT_defconfig
+++ b/configs/P5020DS_NAND_SECURE_BOOT_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_P5020DS=y
 CONFIG_SYS_EXTRA_OPTIONS="RAMBOOT_PBL,NAND,SECURE_BOOT,SYS_TEXT_BASE=0xFFF40000"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y