Makefile: Reduce usage of custom mpc85xx u-boot.bin target

Building of final u-boot.bin binary for mpc85xx via binman is needed only
when inserting DTB binary in the middle of the u-boot ELF binary (before
.bootpg and .resetvec ELF sections).

These requirements are met when CONFIG_MPC85XX_HAVE_RESET_VECTOR is enabled
(= generating .bootpg/.resetvec sections) and CONFIG_OF_SEPARATE is enabled
(= inserting DTB binary).

So in all other cases use standard build procedure instead of custom
mpc85xx u-boot.bin Makefile target via binman.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>