diag: Enable diag over rpmsg communication for adsp

Enable apps to adsp 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: Ia7b9b063130249fad8c7a26ad9bfac543e7567b9
Signed-off-by: Vivek Golani <vgolani@codeaurora.org>