MAINTAINERS: ARM: mvebu: add Sebastian Hesselbarth

Sebastian is a hobbyist who has done a lot of heavy lifting converting
mach-dove to devicetree, and assisting others with patches pertaining to
mvebu.

It is hoped that he will continue this work, and also assist the current
mvebu maintainers with patch wrangling and pull request submissions.

Signed-off-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>