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_SDCARD_defconfig b/configs/P5020DS_SDCARD_defconfig
index 4f6276b..2da6f48 100644
--- a/configs/P5020DS_SDCARD_defconfig
+++ b/configs/P5020DS_SDCARD_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_P5020DS=y
 CONFIG_SYS_EXTRA_OPTIONS="RAMBOOT_PBL,SDCARD,SYS_TEXT_BASE=0xFFF40000"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y