misc: fastrpc: separate fastrpc device from channel context

Currently fastrpc misc device instance is within channel context struct
with a kref. So we have 2 structs with refcount, both of them managing the
same channel context structure.

Separate fastrpc device from channel context and by adding a dedicated
fastrpc_device structure, this should clean the structures a bit and also help
when adding secure device node support.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>