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/[e-mail]
Signed-off-by: Andre Przywara <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>

MAINTAINERS

···
 F:	Documentation/devicetree/bindings/pinctrl/bitmain,bm1880-pinctrl.txt
 
 ARM/CALXEDA HIGHBANK ARCHITECTURE
-M:	Rob Herring <[e-mail]>
+M:	Andre Przywara <[e-mail]>
 L:	[e-mail] (moderated for non-subscribers)
 S:	Maintained
 F:	arch/arm/mach-highbank/