Add a Mailbox interface

Change-Id: If68e70709c496849cb781af5f7288fa3f7ddcdb5
diff --git a/drivers/mailbox/include/mailbox.hpp b/drivers/mailbox/include/mailbox.hpp
new file mode 100644
index 0000000..9074754
--- /dev/null
+++ b/drivers/mailbox/include/mailbox.hpp
@@ -0,0 +1,55 @@
+/*
+ * 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.
+ */
+
+#ifndef MAILBOX_HPP
+#define MAILBOX_HPP
+
+#include <cstddef>
+#include <list>
+
+namespace Mailbox {
+
+class Mailbox {
+public:
+    Mailbox();
+    virtual ~Mailbox();
+    virtual bool sendMessage() = 0;
+    virtual void handleMessage() = 0;
+    virtual bool verifyHardware();
+    typedef void (*CallbackFptr)(void *userArg);
+    void registerCallback(CallbackFptr callback, void *userArg);
+    void deregisterCallback(CallbackFptr callback, void *userArg);
+
+protected:
+    void notify();
+    uint32_t read32(volatile uint32_t *baseAddr, const uint32_t offset);
+    void write32(volatile uint32_t *baseAddr, const uint32_t offset, const uint32_t value);
+
+private:
+    struct Callback {
+        bool operator==(const Callback &b) const;
+        CallbackFptr callback;
+        void *userArg;
+    };
+
+    std::list<Callback> callbacks;
+};
+
+} // namespace Mailbox
+
+#endif