Add documentation for RPMsg Port UART, a transport layer that enables
RPMsg communication between SoCs via UART when shared memory is not
available.
The documentation covers:
- Hardware requirements (UART with flow control)
- Software architecture
- Escape coding protocol for command/data separation
- Connection establishment protocol
- Data frame format
- Low power support with ping-pong wake mechanism
Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>