remoteproc: core: Make DMA-BUF attachment interface thread safe

commit 62b1ab9ed3682b422da077d1c7dce4707b78f898 from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git



Protect dmabuf attach list access with mutex

Signed-off-by: Rahul T R <r-ravikumar@ti.com>
Acked-by: Andrew Davis <afd@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>