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/mailbox/CMakeLists.txt b/drivers/mailbox/CMakeLists.txt
index 6d55407..bd2dae3 100644
--- a/drivers/mailbox/CMakeLists.txt
+++ b/drivers/mailbox/CMakeLists.txt
@@ -16,7 +16,6 @@
 # limitations under the License.
 #
 
-add_library(mailbox STATIC)
-target_include_directories(mailbox PUBLIC include)
-target_sources(mailbox PRIVATE src/mailbox.cc)
-
+add_library(ethosu_mailbox STATIC)
+target_include_directories(ethosu_mailbox PUBLIC include)
+target_sources(ethosu_mailbox PRIVATE src/mailbox.cc)