board: kontron: sl28: add myself to ls1028a.dtsi maintainers

I'd like to keep informed about ls1028a.dtsi changes. For now, there is
no top-level entry for any layerscape specific files. Instead, add the
file entry to my board MAINTAINERS file.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>