arm: highbank: Defeature CPU override

This board should not support CPU model override. This allows for
easier patching of the board with being able to rely on the CPU
type being correct.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 471a61e049c7ca6e82f5ef6668889a1d518c7e00.1447007690.git.crosthwaite.peter@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>