samples/rpmsg: Introduce a module parameter for message count

The current rpmsg_client_sample uses a fixed number of messages to
be sent to each instance. This is currently set at 100. Introduce
an optional module parameter 'count' so that the number of messages
to be exchanged can be made flexible.

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