ipc_rpmsg_static_vrings: use names for WQ threads

This patch adds names to the threads created by `mbox_init` via
`k_work_queue_start`. The name of the thread is the same as the name in
the device instance.

The main reason for this has to do with how `mcumgr` reports [thread
information](https://docs.zephyrproject.org/latest/services/device_mgmt/smp_groups/smp_group_0.html#task-statistics-command

).
Specifically, data about threads is sent in a CBOR encoded map where the
map keys are the thread names (at least in the default configuration). If
there's more than one IPC channel defined (one example of this being
`samples/subsys/logging/multidomain`), both threads would be created
with an empty name and `mcumgr` will only return the data associated
with the thread created by the last call to `mbox_init`.

Signed-off-by: Bogdan Marinescu <bogdan.marinescu@gmail.com>