drivers/rpmsg: add rpmsg router support

Rpmsg Router is new rpmsg transport layer, it can router the rpmsg
messages to a cpu that not directly connected with local cpu by Rpmsg,
For the rpmsg services, it is as if there is a real Rpmsg Channel between
the local cpu and the remote cpu.

For examples, there are three cpus: ap, cp and audio.
ap and cp, ap and audio has share memory and be connected by Rpmsg VirtIO,
so ap and cp, ap and audio can communicate with each other by Rpmsg, but
cp can not communicate with audio direclty.

[cp] <-- rpmsg virtio --> [ap] <-- rpmsg virtio --> [audio]

With rpmsg router, the cp can communicate with audip by Rpmsg dereclty because
the router in ap will forward the rpmsg message from cp/audio to audio/cp, like
this:

 +<----- rpmsg router --> hub  <-- rpmsg router ------>+
 |                         |                           |
[cp] <-- rpmsg virtio --> [ap] <-- rpmsg virtio --> [audio]

Signed-off-by: yintao <yintao@xiaomi.com>

drivers/rpmsg/Kconfig

@@ -23,6 +23,13 @@ config RPMSG_PING
 
 endif # RPMSG
 
+config RPMSG_ROUTER
+	bool "rpmsg router support"
+	default n
+	---help---
+		Rpmsg router driver for enabling communication
+		without physical channels.
+
 config RPMSG_PORT
 	bool
 	default n