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/ea20_defconfig b/configs/ea20_defconfig
index 89d1cfa..6fe19c7 100644
--- a/configs/ea20_defconfig
+++ b/configs/ea20_defconfig
@@ -7,4 +7,5 @@
 CONFIG_CMD_GPIO=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_SYS_NS16550=y