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/B4420QDS_defconfig b/configs/B4420QDS_defconfig
index 34a6219..8308b95 100644
--- a/configs/B4420QDS_defconfig
+++ b/configs/B4420QDS_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_B4860QDS=y
 CONFIG_SYS_EXTRA_OPTIONS="PPC_B4420"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y