MAINTAINERS: net-lwip: add myself as a maintainer

Add myself as a maintainer for the lwIP network stack integration code
and network commands. The library code itself (i.e., most files under
lib/lwip/ except README, Makefile and integration files in u-boot) is
unmodified from upstream and therefore does not need a maintainer.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>