net: wwan: iosm: add rpc interface for xmm modems

Add a new iosm wwan port that connects to the modem rpc interface. This
interface provides a configuration channel, and in the case of the 7360, is
the only way to configure the modem (as it does not support mbim).

The new interface is compatible with existing software, such as
open_xdatachannel.py from the xmm7360-pci project [1].

[1] https://github.com/xmm7360/xmm7360-pci



Signed-off-by: Shane Parslow <shaneparslow808@gmail.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>