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>