MAINTAINERS: Hand MIPS over to Thomas

My time with MIPS the company has reached its end, and so at best I'll
have little time spend on maintaining arch/mips/.

Ralf last authored a patch over 2 years ago, the last time he committed
one is even further back & activity was sporadic for a while before
that. The reality is that he isn't active.

Having a new maintainer with time to do things properly will be
beneficial all round. Thomas Bogendoerfer has been involved in MIPS
development for a long time & has offered to step up as maintainer, so
add Thomas and remove myself & Ralf from the MIPS entry.

Ralf already has an entry in CREDITS to honor his contributions, so this
just adds one for me.

Signed-off-by: Paul Burton <paulburton@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@vger.kernel.org