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/m28evk_defconfig b/configs/m28evk_defconfig
index 9e8508c..9da42eb 100644
--- a/configs/m28evk_defconfig
+++ b/configs/m28evk_defconfig
@@ -5,3 +5,4 @@
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_STMICRO=y