Rpmsg Physical Transport Layer is a new rpmsg transport, it's a
common part for the physical communication based rpmsg transport
layers such as Rpmsg-SPI and Rpmsg-Uart.
It implements three common parts:
1. Implement the NuttX and OpenAMP rpmsg frameworks' ops and the
rpmsg name service;
2. The buffer management and provide some APIs to lower layer to use;
Signed-off-by: liaoao <liaoao@xiaomi.com>
6d604ec487