MAINTAINERS: network: add entry for WWAN

This patch adds maintainer info for drivers/net/wwan subdir, including
WWAN core and drivers. Adding Sergey and myself as maintainers and
Johannes as reviewer.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Acked-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>