arm64: Fix correct dtb clean-files location

This Makefile is evaluated in arch/arm64/boot/Makefile which is what
$(obj) is.

Signed-off-by: Mohamad Ayyash <mkayyash@google.com>
Change-Id: I75355f064e249a8db693e06073f5cf395ca29ab6
Merge remote-tracking branch 'googlecommon/android-3.10' into android-3.10