MIPS: DTS: img: Don't attempt to build-in all .dtb files

When building a FIT image we may want the kernel to build multiple .dtb
files, but we don't want to build them all into the kernel binary as
object files since they'll instead be included in the FIT image.

Commit daa10170 ("MIPS: DTS: img: add device tree for Marduk board")
however created arch/mips/boot/dts/img/Makefile with a line that builds
any enabled .dtb files into the kernel. Remove this & build the
pistachio object specifically, in preparation for adding .dtb targets
which we don't want to build into the kernel.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16484/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/boot/dts/img/Makefile

 dtb-$(CONFIG_MACH_PISTACHIO)	+= pistachio_marduk.dtb
-
-obj-y				+= $(patsubst %.dtb, %.dtb.o, $(dtb-y))
+obj-$(CONFIG_MACH_PISTACHIO)	+= pistachio_marduk.dtb.o
 
 # Force kbuild to make empty built-in.o if necessary
 obj-				+= dummy.o