arm: highbank: Update maintainership

Rob does not have access to any Calxeda systems anymore, also has
expressed a lack of interest in those systems in the past.

I have multiple working Midway nodes under my desk in the office, so
am happy to take over maintainership.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

board/highbank/MAINTAINERS

 HIGHBANK BOARD
-M:	Rob Herring <robh@kernel.org>
+M:	Andre Przywara <andre.przywara@arm.com>
 S:	Maintained
 F:	board/highbank/
 F:	include/configs/highbank.h