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/dfu.c b/drivers/dfu/dfu.c
index 897dfab..6cd3fbb 100644
--- a/drivers/dfu/dfu.c
+++ b/drivers/dfu/dfu.c
@@ -413,6 +413,9 @@
 	} else if (strcmp(interface, "ram") == 0) {
 		if (dfu_fill_entity_ram(dfu, devstr, s))
 			return -1;
+	} else if (strcmp(interface, "sf") == 0) {
+		if (dfu_fill_entity_sf(dfu, devstr, s))
+			return -1;
 	} else {
 		printf("%s: Device %s not (yet) supported!\n",
 		       __func__,  interface);