Makefile: clean image.map

binman tools for creating single image build will create image.map
at the end, which has information about binman image node details.

current u-boot, is unable to clean this image.map so add a command
entry in clean target in Makefile.

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