ARM: dts: remove generated .dtb files on clean
ARM: dts: remove generated .dtb files on clean

commit 5f300acd8ae9f3d4585154370012ffc5c665330f
(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 499cd8298628eeabf0eb5eb6525d4faa0eec80d8
(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 <50c3c575ce2bb12c45e20736ce59e86fe81915b4@ti.com>
Signed-off-by: Olof Johansson <8c69ee23f3f44f8162a64d579c1fa25c2f55298a@lixom.net>

TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas