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/P5020DS_SRIO_PCIE_BOOT_defconfig b/configs/P5020DS_SRIO_PCIE_BOOT_defconfig
index ba20e71..53a279e 100644
--- a/configs/P5020DS_SRIO_PCIE_BOOT_defconfig
+++ b/configs/P5020DS_SRIO_PCIE_BOOT_defconfig
@@ -5,6 +5,7 @@
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y