powerpc: mpc85xx: Only enable binman when it is needed

Quite a few boards using this SoC family don't use binman, yet
CONFIG_BINMAN is enabled for all of them. But the option should only be
enabled if we expect binman to produce an image. Calling binman when the
device tree is missing, etc. will cause failer.

Add a condition so that CONFIG_BINMAN is only enabled as needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>