rpmsg: Make rpmsg_create_ept() take channel_info struct

As we introduce support for additional rpmsg backends, some of these
only supports point-to-point "links" represented by a name. By making
rpmsg_create_ept() take a channel_info struct we allow for these
backends to either be passed a source address, a destination address or
a name identifier.

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