ARM: mx51: Remove mach-cpuimx51sd board file

eukrea_mbimxsd51-baseboard.c and mach-cpuimx51sd.c can be replaced with their
devicetree equivalents: imx51-eukrea-mbimxsd51-baseboard.dts and
imx51-eukrea-cpuimx51.dtsi respectively, so remove the board files.

This allows the conversion of mx51 to a devicetree-only platform.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>