ARM: imx: Remove mx51_babbage board file

The device tree version for the babbage board (imx51-babbage.dtb) has a more
complete support than the board file version, so remove the board file source
code.

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