arm: mps2_an385: Move dts.fixup to board dir

Move the dts.fixup back to board dir in prep for support mps2_an521.
The memory maps between the two mps2_an385 and mps2_an521 differ greatly
so its easier to just keep the fixup files with the board.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>