MAINTAINERS: Remove Simon as Renesas SoC Co-Maintainer

At the end of the v5.3 upstream kernel development cycle, Simon stepped
down from his role as Renesas SoC maintainer.

Remove his maintainership, git repository, and branch from the
MAINTAINERS file, and add an entry to the CREDITS file to honor his
work.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>