powerpc: fix arch/powerpc/dts/Makefile

Since commit 27cb7300 ("Ensure device tree DTS is compiled"),
build succeeds irrespective of the correctness of Makefile.

I am going to revert that commit, so wrong code must be fixed.

CONFIG_MCR3000 is not defined anywhere. CONFIG_TARGET_MCR3000 is the
correct one.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>