net: wwan: t7xx: Add maintainers and documentation

Adds maintainers and documentation for MediaTek t7xx 5G WWAN modem
device driver.

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>