MIPS: DTS: Only build subdir of current platform

Add config check in Makefile to only build the subdir of current platform.

E.g. without this patch:

  AR      arch/mips/built-in.a
  AR      arch/mips/boot/dts/brcm/built-in.a
  AR      arch/mips/boot/dts/cavium-octeon/built-in.a
  AR      arch/mips/boot/dts/img/built-in.a
  AR      arch/mips/boot/dts/ingenic/built-in.a
  AR      arch/mips/boot/dts/lantiq/built-in.a
  DTC     arch/mips/boot/dts/loongson/loongson3_4core_rs780e.dtb
  DTB     arch/mips/boot/dts/loongson/loongson3_4core_rs780e.dtb.S
  AS      arch/mips/boot/dts/loongson/loongson3_4core_rs780e.dtb.o
  DTC     arch/mips/boot/dts/loongson/loongson3_8core_rs780e.dtb
  DTB     arch/mips/boot/dts/loongson/loongson3_8core_rs780e.dtb.S
  AS      arch/mips/boot/dts/loongson/loongson3_8core_rs780e.dtb.o
  AR      arch/mips/boot/dts/loongson/built-in.a
  AR      arch/mips/boot/dts/mscc/built-in.a
  AR      arch/mips/boot/dts/mti/built-in.a
  AR      arch/mips/boot/dts/netlogic/built-in.a
  AR      arch/mips/boot/dts/ni/built-in.a
  AR      arch/mips/boot/dts/pic32/built-in.a
  AR      arch/mips/boot/dts/qca/built-in.a
  AR      arch/mips/boot/dts/ralink/built-in.a
  AR      arch/mips/boot/dts/xilfpga/built-in.a
  AR      arch/mips/boot/dts/built-in.a

With this patch:

  AR      arch/mips/built-in.a
  DTC     arch/mips/boot/dts/loongson/loongson3_4core_rs780e.dtb
  DTB     arch/mips/boot/dts/loongson/loongson3_4core_rs780e.dtb.S
  AS      arch/mips/boot/dts/loongson/loongson3_4core_rs780e.dtb.o
  DTC     arch/mips/boot/dts/loongson/loongson3_8core_rs780e.dtb
  DTB     arch/mips/boot/dts/loongson/loongson3_8core_rs780e.dtb.S
  AS      arch/mips/boot/dts/loongson/loongson3_8core_rs780e.dtb.o
  AR      arch/mips/boot/dts/loongson/built-in.a
  AR      arch/mips/boot/dts/built-in.a
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/dts/Makefile

 # SPDX-License-Identifier: GPL-2.0
-subdir-y	+= brcm
-subdir-y	+= cavium-octeon
-subdir-y	+= img
-subdir-y	+= ingenic
-subdir-y	+= lantiq
-subdir-y	+= loongson
-subdir-y	+= mscc
-subdir-y	+= mti
-subdir-y	+= netlogic
-subdir-y	+= ni
-subdir-y	+= pic32
-subdir-y	+= qca
-subdir-y	+= ralink
-subdir-y	+= xilfpga
+subdir-$(CONFIG_BMIPS_GENERIC)		+= brcm
+subdir-$(CONFIG_CAVIUM_OCTEON_SOC)	+= cavium-octeon
+subdir-$(CONFIG_MACH_PISTACHIO)		+= img
+subdir-$(CONFIG_MACH_INGENIC)		+= ingenic
+subdir-$(CONFIG_LANTIQ)			+= lantiq
+subdir-$(CONFIG_MACH_LOONGSON64)	+= loongson
+subdir-$(CONFIG_MSCC_OCELOT)		+= mscc
+subdir-$(CONFIG_MIPS_MALTA)		+= mti
+subdir-$(CONFIG_NLM_XLP_BOARD)		+= netlogic
+subdir-$(CONFIG_FIT_IMAGE_FDT_NI169445)	+= ni
+subdir-$(CONFIG_MACH_PIC32)		+= pic32
+subdir-$(CONFIG_ATH79)			+= qca
+subdir-$(CONFIG_RALINK)			+= ralink
+subdir-$(CONFIG_FIT_IMAGE_FDT_XILFPGA)	+= xilfpga
 
 obj-$(CONFIG_BUILTIN_DTB)	:= $(addsuffix /, $(subdir-y))