arm64: add config option to enable DT overlay

Add config option to enable DT overlay. This option sets DTC_FLAGS to
enable DT compilation with overlay support.

Change-Id: I74b1ed9828ec94093fab40a70c0cd0c155796c94
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>
Signed-off-by: Puja Gupta <pujag@codeaurora.org>
Signed-off-by: Runmin Wang <runminw@codeaurora.org>