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: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>