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/T4240QDS_SRIO_PCIE_BOOT_defconfig b/configs/T4240QDS_SRIO_PCIE_BOOT_defconfig
index f6353ca..0adca74 100644
--- a/configs/T4240QDS_SRIO_PCIE_BOOT_defconfig
+++ b/configs/T4240QDS_SRIO_PCIE_BOOT_defconfig
@@ -5,6 +5,7 @@
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y