hafnium: add device tree compiler

Hafnium's build requires the device tree compiler tool.
Until now the dtc binary was provided by the project's prebuilts
submodule. Moving forward prefer sourcing the tool from the docker
image.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: Ib740efbb32f21b92c4c882033bb2d0fa4f4358ad
diff --git a/focal-amd64-hafnium/Dockerfile b/focal-amd64-hafnium/Dockerfile
index 3ae1e62..6217f20 100644
--- a/focal-amd64-hafnium/Dockerfile
+++ b/focal-amd64-hafnium/Dockerfile
@@ -25,6 +25,7 @@
     strace \
     sudo \
     wget \
+    device-tree-compiler \
 "
 
 # Can be overriden at build time