powerpc/mpc85xx: Unset CONFIG_SPL_TARGET="u-boot-with-spl.bin"

powerpc/mpc85xx: Unset CONFIG_SPL_TARGET="u-boot-with-spl.bin"

CONFIG_SPL_TARGET should specify additional SPL make target. But
u-boot-with-spl.bin is final U-Boot binary, not SPL binary in some custom
format. Moreover u-boot-with-spl.bin is already set in CONFIG_BUILD_TARGET,
so make will build it by default.

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