platform/arm: propagate model parameter

VExpress platforms were not updated to handle "model" parameter when it
was added to Platfrom.
diff --git a/devlib/platform/arm.py b/devlib/platform/arm.py
index 76b58a4..7d3ced2 100644
--- a/devlib/platform/arm.py
+++ b/devlib/platform/arm.py
@@ -33,6 +33,7 @@
                  core_names=None,
                  core_clusters=None,
                  big_core=None,
+                 model=None,
                  modules=None,
 
                  # serial settings
@@ -61,6 +62,7 @@
                                                        core_names,
                                                        core_clusters,
                                                        big_core,
+                                                       model,
                                                        modules)
         self.serial_port = serial_port
         self.baudrate = baudrate