misc: fastrpc: Avoid free of DMA buffer in interrupt context

When the remote DSP invocation is interrupted by the user, the
associated DMA buffer can be freed in interrupt context causing a kernel
BUG.

This patch adds a worker thread associated to the fastrpc context. It
is scheduled in the rpmsg callback to decrease its refcount out of the
interrupt context.

Fixes: c68cfb71 ("misc: fastrpc: Add support for context Invoke method")
Signed-off-by: Thierry Escande <thierry.escande@linaro.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>