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/MPC8536DS_SDCARD_defconfig b/configs/MPC8536DS_SDCARD_defconfig
index 16dfab5..a7492fd 100644
--- a/configs/MPC8536DS_SDCARD_defconfig
+++ b/configs/MPC8536DS_SDCARD_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_MPC8536DS=y
 CONFIG_SYS_EXTRA_OPTIONS="SDCARD"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y