Add a concise RPMsg documentation covering:
- Overview of RPMsg framework for AMP systems
- Application scenarios (heterogeneous/homogeneous AMP)
- Layered architecture (Services/Framework/Transport/Physical)
- Message encapsulation process
- Workflow: channel establishment, sending and receiving
- Key design considerations (FIFO order, callback blocking)
- Transport layer comparison
All diagrams use ASCII art for portability.
Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>
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