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/P3041DS_defconfig b/configs/P3041DS_defconfig
index bc453cc..b41254d 100644
--- a/configs/P3041DS_defconfig
+++ b/configs/P3041DS_defconfig
@@ -2,6 +2,7 @@
 CONFIG_MPC85xx=y
 CONFIG_TARGET_P3041DS=y
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y