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/P2041RDB_NAND_defconfig b/configs/P2041RDB_NAND_defconfig
index 6295b53..540b79d 100644
--- a/configs/P2041RDB_NAND_defconfig
+++ b/configs/P2041RDB_NAND_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_P2041RDB=y
 CONFIG_SYS_EXTRA_OPTIONS="RAMBOOT_PBL,NAND,SYS_TEXT_BASE=0xFFF40000"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y