MAINTAINERS: Remove Kirkwood

arch/arm/mach-kirkwood has been removed, since kirkwood is now
supported by arch/arm/mach-mvebu. Remove it from the MAINTAINERS file.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Olof Johansson <olof@lixom.net>