Bluetooth: drivers/rpmsg: Ensure coop prio bt_recv

Ensure that the Bluetooth rpmsg driver thread is temporarly in
cooperative mode using `k_sched_lock`.

The previous commit added the documentation on `bt_recv` stemming from
the general consensus among maintainers that `bt_recv` may not be called
from preemptible priorites.

Many uses may be affected by this race condition, since the default
configuration of rpmsg driver selects a preemtible priority.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>