ARM: imx5: drop option MACH_IMX51_DT

Since i.MX51 becomes DT only now, we can drop option MACH_IMX51_DT and
just use SOC_IMX51 instead.  While at it, rename imx51-dt.c to
mach-imx51.c to align with the name schema of other IMX DT only
platforms.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>