arm64: Add dtb files to archclean rule

As dts files have been reorganised under vendor subdirs, dtb files
cannot be removed with "make distclean" now. Thus, this patch moves
dtb files under archclean rule and removes unnecessary entries.

Cc: Robert Richter <rrichter@cavium.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: NJungseok Lee <jungseoklee85@gmail.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/Makefile

@@ -85,6 +85,7 @@ vdso_install:
 # We use MRPROPER_FILES and CLEAN_FILES now
 archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)
+	$(Q)$(MAKE) $(clean)=$(boot)/dts
 
 define archhelp
   echo  '* Image.gz      - Compressed kernel image (arch/$(ARCH)/boot/Image.gz)'

arch/arm64/boot/dts/Makefile

@@ -3,6 +3,4 @@ dts-dirs += apm
 dts-dirs += arm
 dts-dirs += cavium
 
-always		:= $(dtb-y)
 subdir-y	:= $(dts-dirs)
-clean-files	:= *.dtb