Makefile: clean bl31_*.bin

Rockchip platform has its python script that would generate various
bl31_*bin for creating u-boot.itb file by taking bl31.elf as input.

These bl31_*.bin files are generated in u-boot root directory and
have no rule to clean it up. so add support for it by adding in
command entry of clean target in Makefile.
Signed-off-by: NJagan Teki <jagan@amarulasolutions.com>
Reviewed-by: NKever Yang <kever.yang@rock-chips.com>

Makefile

@@ -1848,7 +1848,7 @@ clean: $(clean-dirs)
 		-o -name 'dsdt.aml' -o -name 'dsdt.asl.tmp' -o -name 'dsdt.c' \
 		-o -name '*.efi' -o -name '*.gcno' -o -name '*.so' \) \
 		-type f -print | xargs rm -f \
-		image.map
+		bl31_*.bin image.map
 
 # mrproper - Delete all generated files, including .config
 #