rpmsg: Move rpmsg_device API to new file

Extract the now indirect rpmsg_create_ept() interface to a separate
file and start building up a rpmsg core.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
diff --git a/drivers/rpmsg/Kconfig b/drivers/rpmsg/Kconfig
index 69a2193..40614be8 100644
--- a/drivers/rpmsg/Kconfig
+++ b/drivers/rpmsg/Kconfig
@@ -3,6 +3,10 @@
 # RPMSG always gets selected by whoever wants it
 config RPMSG
 	tristate
+
+config RPMSG_VIRTIO
+	tristate
+	select RPMSG
 	select VIRTIO
 	select VIRTUALIZATION