dtc: add submodule

Add dtc submodule as a fallback for old distros.
Picking version 1.3.0. as this is the most recently tagged stable version.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>

.gitmodules

@@ -22,3 +22,6 @@
 [submodule "pixman"]
 	path = pixman
 	url = git://anongit.freedesktop.org/pixman
+[submodule "dtc"]
+	path = dtc
+	url = git://git.qemu.org/dtc.git

dtc @ bc895d6d

+Subproject commit bc895d6d09695d05ceb8b52486ffe861d6cfbdde