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/applications/message_process/CMakeLists.txt b/applications/message_process/CMakeLists.txt
index 9c80672..2f7e5cf 100644
--- a/applications/message_process/CMakeLists.txt
+++ b/applications/message_process/CMakeLists.txt
@@ -18,4 +18,4 @@
 
 add_library(message_process STATIC src/message_process.cc)
 target_include_directories(message_process PUBLIC include ${LINUX_DRIVER_STACK_PATH}/kernel)
-target_link_libraries(message_process PRIVATE cmsis_core inference_process mailbox)
+target_link_libraries(message_process PRIVATE cmsis_core inference_process ethosu_mailbox)