diag: Use mutex instead of spinlock for synchronization

Usage of spinlock for rpmsg send call in rpmsg write leads to sleep
issue while in atomic context. Replace spinlock inplace of mutex to
synchronize rpmsg read and write.

Change-Id: I6384961fcbb25bca21781c2b88353e141f1614bc
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>