diag: Enable diag over rpmsg communication

Enable apps to wdsp diag communication over rpmsg. Changes were
made to copy the data from glink 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: I53681785a72046bb5d0e9fa7461b97c0f776d412
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>