diag: Synchronize rpmsg read and write

Use per peripheral channel_mutex to synchronize the rpmsg read
and write to avoid race condition and use per channel and per
peripheral buffers effectively.

Change-Id: Ida08db6776f03cb729a24b15b445fd726e25a153
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>