dfu: add SF backend

This allows SPI Flash to be programmed using DFU.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
diff --git a/drivers/dfu/Makefile b/drivers/dfu/Makefile
index def628d..5cc535e 100644
--- a/drivers/dfu/Makefile
+++ b/drivers/dfu/Makefile
@@ -9,3 +9,4 @@
 obj-$(CONFIG_DFU_MMC) += dfu_mmc.o
 obj-$(CONFIG_DFU_NAND) += dfu_nand.o
 obj-$(CONFIG_DFU_RAM) += dfu_ram.o
+obj-$(CONFIG_DFU_SF) += dfu_sf.o