bus: mhi: host: pci_generic: Add a secondary AT port to Telit FN990

Add a secondary AT port using one of OEM reserved channel.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://lore.kernel.org/r/20220916144329.243368-3-fabio.porcedda@gmail.com
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>