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
Git-commit: 092f5269621cc6654c9b77ffde7a241cb65d0e50
Git-repo: https://android.googlesource.com/kernel/common.git


Signed-off-by: Ian Maund <imaund@codeaurora.org>