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/P1020RDB-PC_36BIT_defconfig b/configs/P1020RDB-PC_36BIT_defconfig
index 32b7008..3fad0b9 100644
--- a/configs/P1020RDB-PC_36BIT_defconfig
+++ b/configs/P1020RDB-PC_36BIT_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_P1_P2_RDB_PC=y
 CONFIG_SYS_EXTRA_OPTIONS="P1020RDB_PC,36BIT"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y