Add basic MHU drivers

Add MHUv2 doorbell driver implementation
Add MHU doorbell driver for Juno board
Add a dummy MHU driver
Add some comments to the mailbox header file

Change-Id: I0950a7ca3afeec88ca691ca2486022dfbb3319b8
diff --git a/drivers/mhu_juno/CMakeLists.txt b/drivers/mhu_juno/CMakeLists.txt
new file mode 100644
index 0000000..1dbb6fd
--- /dev/null
+++ b/drivers/mhu_juno/CMakeLists.txt
@@ -0,0 +1,23 @@
+#
+# Copyright (c) 2020 Arm Limited. All rights reserved.
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the License); you may
+# not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an AS IS BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+add_library(ethosu_mhu_juno STATIC)
+target_include_directories(ethosu_mhu_juno PUBLIC include)
+
+target_sources(ethosu_mhu_juno PRIVATE src/mhu_juno.cc)
+target_link_libraries(ethosu_mhu_juno PRIVATE ethosu_mailbox)