rpmsg: add THIS_MODULE to rpmsg_driver in rpmsg core

Add register_rpmsg_driver helper macro that adds THIS_MODULE to
rpmsg_driver for the registering driver. We rename and modify
the existing register_rpmsg_driver to enable this.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>