ARM: dt: Also clean dtbo files

dtbo files can be also generated that's why also clean them.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/arch/arm/dts/Makefile b/arch/arm/dts/Makefile
index d9aaf12..e67ff32 100644
--- a/arch/arm/dts/Makefile
+++ b/arch/arm/dts/Makefile
@@ -833,4 +833,4 @@
 dtbs: $(addprefix $(obj)/, $(dtb-y))
 	@:
 
-clean-files := *.dtb *_HS
+clean-files := *.dtb *.dtbo *_HS