rpmsg: smd: Register rpmsg user space interface for edges

Create and register a rpmsg device for use with the rpmsg user space
interface, allowing user space to access SMD channels.

Also provide the "rpmsg_name" device attribute to expose the edge name
in sysfs, allowing the user to write udev rules for specific rpmsg
devices and their children.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>