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/bf526-ezbrd_defconfig b/configs/bf526-ezbrd_defconfig
index 6a652a0..a5f4b73 100644
--- a/configs/bf526-ezbrd_defconfig
+++ b/configs/bf526-ezbrd_defconfig
@@ -4,3 +4,4 @@
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y