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: NMasahiro Yamada <yamada.masahiro@socionext.com>

arch/powerpc/dts/Makefile

 # SPDX-License-Identifier: GPL-2.0+
 
 dtb-$(CONFIG_TARGET_T2080QDS) += t2080qds.dtb
-dtb-$(CONFIG_MCR3000) += mcr3000.dtb
+dtb-$(CONFIG_TARGET_MCR3000) += mcr3000.dtb
 
 targets += $(dtb-y)