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/P1025RDB_36BIT_defconfig b/configs/P1025RDB_36BIT_defconfig
index c98ec95..453c057 100644
--- a/configs/P1025RDB_36BIT_defconfig
+++ b/configs/P1025RDB_36BIT_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_P1_P2_RDB_PC=y
 CONFIG_SYS_EXTRA_OPTIONS="P1025RDB,36BIT"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y