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: NMichael Walle <michael@walle.cc>
Reviewed-by: NPriyanka Jain <priyanka.jain@nxp.com>

board/kontron/sl28/MAINTAINERS

 Kontron SMARC-sAL28 board
 M:	Michael Walle <michael@walle.cc>
 S:	Maintained
+F:	arch/arm/dts/fsl-ls1028a.dtsi
 F:	arch/arm/dts/fsl-ls1028a-kontron-sl28-*
 F:	board/kontron/sl28/
 F:	configs/kontron_sl28_defconfig