Add MHI/WWAN support

The MHI support for control channels/ports has recently been merged in Linux along with a new 'wwan subsystem'. The wwan subsystem handles wwan devices (e.g. /sys/class/wwan0) and related ports (e.g. /sys/class/wwan0/wwan0p1MBIM...). Each WWAN control port is exposed as a character device for usermode access to control protocols (QMI, MBIM, AT...), and so can be used by ModemManager as standard modem port.

On the other side mhi_net is the driver for the network data path, exposed as a standard netdev. the mhi_net netdev exposes either as a IP/QMAP(rmnet) interface (e.g. mhi_hwip0) or a MBIM interface (e.g mhi_mbim0). In the case of IP/QMAP, the netdev can be used either to transport raw IP data (no aggregation protocol) or QMAP data (and so needs extra upper rmnet link/netdev to be configured). In case of MBIM, interface is exposing raw IP and MBIM protocol is directly handled inside mhi_net driver. Since it's a simplified version of MBIM. multiplexing is not yet supported, and all data received by mhi_mbim0 is directed to session 0.