MAINTAINERS: Update Calxeda Highbank maintainership

Rob sees little point in maintaining the Calxeda architecture (early ARM
32-bit server) anymore.
Since I have a machine sitting under my desk, change the maintainership
to not lose support for that platform.

Link: https://lore.kernel.org/r/20200228135106.220620-6-andre.przywara@arm.com


Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>