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.