ARM: mx25: Remove "mx25.h" header file

As mx25 has been converted to a dt-only platform, we do not need the "mx25.h"
header file anymore.

Remove it and also clean up all the mx25 occurences from the platform helper
code.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>