cdx: add rpmsg communication channel for CDX

RPMsg is used as a transport communication channel. This
change introduces RPMsg driver and integrates it with the
CDX controller.

Signed-off-by: Abhijit Gangurde <abhijit.gangurde@amd.com>
Signed-off-by: Nipun Gupta <nipun.gupta@amd.com>
Reviewed-by: Pieter Jansen van Vuuren <pieter.jansen-van-vuuren@amd.com>
Tested-by: Nikhil Agarwal <nikhil.agarwal@amd.com>
Link: https://lore.kernel.org/r/20230313132636.31850-7-nipun.gupta@amd.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>