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>
diff --git a/arch/arm/mach-imx/Makefile b/arch/arm/mach-imx/Makefile
index bbe93bb..9d7b46f 100644
--- a/arch/arm/mach-imx/Makefile
+++ b/arch/arm/mach-imx/Makefile
@@ -109,8 +109,8 @@
 endif
 obj-$(CONFIG_SOC_IMX6) += pm-imx6.o
 
-obj-$(CONFIG_MACH_IMX51_DT) += imx51-dt.o
 obj-$(CONFIG_SOC_IMX50) += mach-imx50.o
+obj-$(CONFIG_SOC_IMX51) += mach-imx51.o
 obj-$(CONFIG_SOC_IMX53) += mach-imx53.o
 
 obj-$(CONFIG_SOC_VF610) += clk-vf610.o mach-vf610.o