microblaze: Remove *.dtb files in make clean

dts/Makefile is called only for simpleImage target
which is causing that *.dtb are not removed.
This patch fix it.
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/boot/Makefile

@@ -34,5 +34,4 @@ $(obj)/simpleImage.%: vmlinux FORCE
 	$(call if_changed,strip)
 	@echo 'Kernel: $@ is ready' ' (#'`cat .version`')'
 
-
-clean-files += simpleImage.*.unstrip linux.bin.ub
+clean-files += simpleImage.*.unstrip linux.bin.ub dts/*.dtb

arch/microblaze/boot/dts/Makefile

@@ -16,5 +16,3 @@ quiet_cmd_cp = CP      $< $@$2
 
 # Rule to build device tree blobs
 DTC_FLAGS := -p 1024
-
-clean-files += *.dtb