Remove "atmel_df_pow2" binary with "make clean"

Commit 65f6f07b added support for the atmel_df_pow2 standalone program
but missed to add a rule to remove it to the "clean" make target.

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/Makefile b/Makefile
index 0449a5b..54610c6 100644
--- a/Makefile
+++ b/Makefile
@@ -3717,6 +3717,7 @@
 
 clean:
 	@rm -f $(obj)examples/standalone/82559_eeprom			  \
+	       $(obj)examples/standalone/atmel_df_pow2			  \
 	       $(obj)examples/standalone/eepro100_eeprom		  \
 	       $(obj)examples/standalone/hello_world			  \
 	       $(obj)examples/standalone/interrupt			  \