arm: dts: Makefile: clean *dtb_HS

TI HS platforms generate *dtb_HS binary blobs and there is no
rule for cleanup. Added entry for cleanup in clean-files target.

Signed-off-by: Suniel Mahesh <sunil.m@techveda.org>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
diff --git a/arch/arm/dts/Makefile b/arch/arm/dts/Makefile
index ebe5495..05ff624 100644
--- a/arch/arm/dts/Makefile
+++ b/arch/arm/dts/Makefile
@@ -807,4 +807,4 @@
 dtbs: $(addprefix $(obj)/, $(dtb-y))
 	@:
 
-clean-files := *.dtb
+clean-files := *.dtb *_HS