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_SPIFLASH_defconfig b/configs/P5020DS_SPIFLASH_defconfig
index 9dc2d0b..ae06e9c 100644
--- a/configs/P5020DS_SPIFLASH_defconfig
+++ b/configs/P5020DS_SPIFLASH_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_P5020DS=y
 CONFIG_SYS_EXTRA_OPTIONS="RAMBOOT_PBL,SPIFLASH,SYS_TEXT_BASE=0xFFF40000"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y