ARM: dts: remove generated .dtb files on clean

commit 5f300acd
(ARM: 7152/1: distclean: Remove generated .dtb files)
ensured that dtbs were cleaned up when they were in
arch/arm/boot.
However, with the following commit:
commit 499cd829
(ARM: dt: change .dtb build rules to build in dts directory)

make clean now leaves dtbs in arch/arm/boot/dts/
untouched. Include dts directory so that clean-files rule
from arch/arm/boot/dts/Makefile is invoked when make
clean is done.

Cc: Dirk Behme <dirk.behme@de.bosch.com>
CC: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Olof Johansson <olof@lixom.net>

arch/arm/boot/Makefile

@@ -115,4 +115,4 @@ i:
 	$(CONFIG_SHELL) $(srctree)/$(src)/install.sh $(KERNELRELEASE) \
 	$(obj)/Image System.map "$(INSTALL_PATH)"
 
-subdir-	    := bootp compressed
+subdir-	    := bootp compressed dts