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/T2081QDS_defconfig b/configs/T2081QDS_defconfig
index c20b556..23ed67c 100644
--- a/configs/T2081QDS_defconfig
+++ b/configs/T2081QDS_defconfig
@@ -3,6 +3,9 @@
 CONFIG_TARGET_T208XQDS=y
 CONFIG_SYS_EXTRA_OPTIONS="PPC_T2081"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_EON=y
+CONFIG_SPI_FLASH_STMICRO=y
+CONFIG_SPI_FLASH_SST=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y