arch/mips/boot/dts/Makefile collects objects from sub-directories into
built-in.a only when CONFIG_BUILTIN_DTB is enabled. Reflect it also to
the sub-directory Makefiles. This suppresses unneeded built-in.a
creation in arch/mips/boot/dts/*/ directories.

While I am here, I replaced $(patsubst %.dtb, %.dtb.o, $(dtb-y)) with
$(addsuffix .o, $(dtb-y)) to simplify the code a little bit.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Cercueil <paul@crapouillou.net>
Cc: Mathieu Malaterre <malat@debian.org>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/19099/Signed-off-by: NJames Hogan <jhogan@kernel.org>