MAINTAINERS: update NETWORK and NETWORK (LWIP)

MAINTAINERS: update NETWORK and NETWORK (LWIP)

Add myself as a maintainer of the NETWORK subsystem since:
- I have effectively been handling net patches in my patchwork queue and
sending pull requests to Tom,
- I do have push access to the u-boot-net custodian tree.

Also, add u-boot-net as the SCM tree for NETWORK (LWIP) since it is where
lwIP-related patches end up too.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
CC: Ramon Fried <rfried.dev@gmail.com>
CC: Joe Hershberger <joe.hershberger@ni.com>
CC: Tom Rini <trini@konsulko.com>