diag: Enable diag over rpmsg communication for modem

Enable apps to modem diag communication over rpmsg. Changes were
made to copy the data from smd buffer into diag buffer in the
callback and queue a work to process the data in work queue and
release the diag buffer in work queue.

Change-Id: I4a963eb7f3714100419068103712a22c5d5c0c95
Signed-off-by: Vivek Golani <vgolani@codeaurora.org>