MAINTAINERS: net: lwip: add myself as a maintainer

Add myself as a maintainer for the lwIP network stack integration code
and network commands as well as the sandbox ethernet driver for lwIP.

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