ARM: mx25: Remove mach-mx25_3ds board file

imx25-pdk.dts provides a more complete support than the board file version, so
let's get rid of the board file.

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