MIPS: have "plain" make calls build dtbs for selected platforms

scripts/package/builddeb calls "make dtbs_install" after executing
a plain make (i.e. no build targets specified). It will fail if dtbs
were not built beforehand. Match the arm64 architecture where DTBs get
built by the "all" target.
Signed-off-by: Cedric Hombourger <Cedric_Hombourger@mentor.com>
[paul.burton@mips.com: s/builddep/builddeb]
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: stable@vger.kernel.org # v4.1+

arch/mips/Makefile

@@ -17,6 +17,7 @@ archscripts: scripts_basic
 	$(Q)$(MAKE) $(build)=arch/mips/boot/tools relocs
 
 KBUILD_DEFCONFIG := 32r2el_defconfig
+KBUILD_DTBS      := dtbs
 
 #
 # Select the object file format to substitute into the linker script.
@@ -384,7 +385,7 @@ quiet_cmd_64 = OBJCOPY $@
 vmlinux.64: vmlinux
 	$(call cmd,64)
 
-all:	$(all-y)
+all:	$(all-y) $(KBUILD_DTBS)
 
 # boot
 $(boot-y): $(vmlinux-32) FORCE