diag: Replace mutex with spinlock for synchronization

Synchronize rpmsg info init variables with spinlock instead of
mutex in an atomic context while freeing up the peripheral buffers.

Change-Id: Ia25c312c0a7c60d032615388758c80a06c3b78d7
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>