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/bg0900_defconfig b/configs/bg0900_defconfig
index 2cb8831..620d621 100644
--- a/configs/bg0900_defconfig
+++ b/configs/bg0900_defconfig
@@ -6,3 +6,4 @@
 CONFIG_CMD_GPIO=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_BAR=y
+CONFIG_SPI_FLASH_STMICRO=y