net: wwan: t7xx: Add WWAN network interface

Creates the Cross Core Modem Network Interface (CCMNI) which implements
the wwan_ops for registration with the WWAN framework, CCMNI also
implements the net_device_ops functions used by the network device.
Network device operations include open, close, start transmission, TX
timeout and change MTU.

Signed-off-by: Haijun Liu <haijun.liu@mediatek.com>
Co-developed-by: Chandrashekar Devegowda <chandrashekar.devegowda@intel.com>
Signed-off-by: Chandrashekar Devegowda <chandrashekar.devegowda@intel.com>
Co-developed-by: Ricardo Martinez <ricardo.martinez@linux.intel.com>
Signed-off-by: Ricardo Martinez <ricardo.martinez@linux.intel.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>