RPi: Add mbrugger as board maintainer

I took over maintainership from Alex Graf with commit
3157bbfa18 ("rpi: Make Matthias maintainer")
But I forgot to update the board maintainer file.
This patch adds myself to the game.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Alexander Graf <agraf@csgraf.de>
1 file changed