samples/rpmsg: add support for multiple instances

The current rpmsg_client_sample is a very simple example and
is not designed to handle multiple instances. Add support for
multiple instances, so that the same number of pings are sent
to each instance. The instances can be on one or multiple
remote processors.

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