ipc: ipc_service: icbmsg backend: workaround endpoint binding deadlock

This change works around the issue with the semaphore timeout during
the Bluetooth HCI driver initialization when the bt_enable function
is called in the context of the System Workqueue thread. This issue
only affects platform that use the IPC service and its ICBMsg backend
(e.g. the nRF54H20 DK target).

The bt_enable function, when called in the System Workqueue context,
results in a deadlock, as the waiting semaphore of the Bluetooth HCI
driver times out:

bt_hci_driver: Endpoint binding failed with -11

During the Bluetooth HCI driver open operation in the context of the
bt_enable function, the driver code waits using the semaphore for the
endpoint binding process of the IPC service module to finalize. The
issue occurs when the  waiting occurs in the System Workqueue context.
The ICBMsg backend from the IPC service schedules a system work during
the endpoint registration, in which it finalizes the binding operation
- also in the System Workqueue context. As the Bluetooth HCI driver
with its wait operation keeps the System Workqueue context busy, the
endpoint binding cannot be completed by the ICBMsg backend before the
HCI driver semaphore timeout.

Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>