ARM: mx25: Convert to a dt-only platform

As there is no more mx25 board files, we can turn mx25 into a dt-only platform.

Rename imx25-dt.c to mach-imx25.c to be consistent with the other i.MX SoCs.

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