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/Makefile b/drivers/rpmsg/Makefile
index 7617fcb..c48ea55 100644
--- a/drivers/rpmsg/Makefile
+++ b/drivers/rpmsg/Makefile
@@ -1 +1,2 @@
-obj-$(CONFIG_RPMSG)	+= virtio_rpmsg_bus.o
+obj-$(CONFIG_RPMSG)		+= rpmsg_core.o
+obj-$(CONFIG_RPMSG_VIRTIO)	+= virtio_rpmsg_bus.o