socfpga: add Simon Goldschmidt as co-custodian

This updates MAINTAINERS and git-mailrc to add me as a
co-custodian for socfpga.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Marek Vasut <marex@denx.de>