rpmsg: rpc: maintain a reference device pointer per open fd

The remote processor recovery process includes the deletion and
recreation of an rpmsg-rpc device. The representative rppc_device
structure is retained and reused if there are any open applications
using the exposed character device. The underlying device pointer
for a rppc_device is though deleted and recreated and is asynchronous
to any of the operations on the exposed character device. A reference
to this device pointer is to be maintained therefore for each open
application so that it can be used during regular fops and until the
file descriptor is closed instead of referencing the rppc_device's
dev pointer, which can become NULL at any point due to a recovery
process. The actual memory of the rppc_device's dev pointer deleted
in the driver's .remove() is freed when all the open applications
have closed either gracefully or forcefully. Any new applications
after a recovery will leverage a newly created device pointer.

Signed-off-by: Suman Anna <s-anna@ti.com>