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>

Makefile

@@ -3717,6 +3717,7 @@ grsim_leon2_config : unconfig
 
 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			  \