Add base class for memory-mapped I/O devices.

mmiodevice.h

@@ -0,0 +1,14 @@
+#ifndef MMIO_DEVICE_H
+#define MMIO_DEVICE_H
+
+#include <cinttypes>
+
+/** Abstract class representing a simple, memory-mapped I/O device */
+class MMIODevice {
+public:
+    virtual uint32_t read(uint32_t offset, int size) = 0;
+    virtual void     write(uint32_t offset, uint32_t value, int size) = 0;
+    virtual ~MMIODevice() = default;
+};
+
+#endif /* MMIO_DEVICE_H */