powerpc: mpc85xx: Fix CONFIG_OF_EMBED support for NOR booting

mpc85xx NOR binary contains also reset vector and therefore option
CONFIG_MPC85XX_HAVE_RESET_VECTOR must be defined.

When build system uses binman, it takes care of constructing final image
which consist of u-boot-without-reset-vector, DTB and reset-vector.

CONFIG_OF_EMBED does not use binman, there is no external DTB and Makefile
produce directly final u-boot.bin binary.

So in this case mpc85xx reset vector must not be stripped from the final
u-boot.bin binary. Fix it.

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