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-PD_defconfig b/configs/P1020RDB-PD_defconfig
index 295cfaf..3c97423 100644
--- a/configs/P1020RDB-PD_defconfig
+++ b/configs/P1020RDB-PD_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_P1_P2_RDB_PC=y
 CONFIG_SYS_EXTRA_OPTIONS="P1020RDB_PD"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y