dm: sf: Add tests for SPI flash

Add a simple test for SPI that uses SPI flash. It operates by creating a
SPI flash file and using the 'sf test' command to test that all
operations work correctly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index d1b9c9a..75d3d41 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -19,4 +19,5 @@
 ifneq ($(CONFIG_SANDBOX),)
 obj-$(CONFIG_DM_GPIO) += gpio.o
 obj-$(CONFIG_DM_SPI) += spi.o
+obj-$(CONFIG_DM_SPI_FLASH) += sf.o
 endif