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/dra7xx_evm_uart3_defconfig b/configs/dra7xx_evm_uart3_defconfig
index b4fadd4..204483a 100644
--- a/configs/dra7xx_evm_uart3_defconfig
+++ b/configs/dra7xx_evm_uart3_defconfig
@@ -12,5 +12,6 @@
 # CONFIG_CMD_SETEXPR is not set
 # CONFIG_CMD_NFS is not set
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_SYS_NS16550=y
 CONFIG_TI_QSPI=y