rpmsg: rpmsg_send() operations takes rpmsg_endpoint

The rpmsg_send() operations has been taking a rpmsg_device, but this
forces users of secondary rpmsg_endpoints to use the rpmsg_sendto()
interface - by extracting source and destination from the given data
structures. If we instead pass the rpmsg_endpoint to these functions a
service can use rpmsg_sendto() to respond to messages, even on secondary
endpoints.

In addition this would allow us to support operations on multiple
channels in future backends that does not support off-channel
operations.

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