misc: fastrpc: Reference count channel context

The channel context is referenced from the fastrpc user and might as
user space holds the file descriptor open outlive the fastrpc device,
which is removed when the remote processor is shutting down.

Reference count the channel context in order to retain this object until
all references has been relinquished.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Mayank Chopra <mak.chopra@codeaurora.org>
Signed-off-by: Abhinav Asati <asatiabhi@codeaurora.org>
Signed-off-by: Vamsi Singamsetty <vamssi@codeaurora.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>