arm/boot: move highbank secure board setup code to common routine

The new version is slightly different, to support Rasbperry Pi (in
particular, Pi1's arm11 core which doesn't support v7 instructions
such as MOVW).

Tested-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>