mach-mx51_3ds only supports old silicon version of MX51 and was replaced
with mx51 babbage, which is the official MX51 development board.

No need to maintain it anymore.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>