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/B4860QDS_NAND_defconfig b/configs/B4860QDS_NAND_defconfig
index 1e2ccc4..a8f05db 100644
--- a/configs/B4860QDS_NAND_defconfig
+++ b/configs/B4860QDS_NAND_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="PPC_B4860,RAMBOOT_PBL,SPL_FSL_PBL,NAND"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y