ARM: imx5x: clean up ARCH_MX5X

Move to SOC_SOC_IMX5X. Leave only places which prevent multi-soc
using ARCH_MX5X.

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
diff --git a/arch/arm/mach-mx5/Kconfig b/arch/arm/mach-mx5/Kconfig
index 03ec6e9..83ee088 100644
--- a/arch/arm/mach-mx5/Kconfig
+++ b/arch/arm/mach-mx5/Kconfig
@@ -1,5 +1,6 @@
 if ARCH_MX5
-# ARCH_MX51 and ARCH_MX50 are left for compatibility
+# ARCH_MX50/51/53 are left to mark places where prevent multi-soc in single
+# image. So for most time, SOC_IMX50/51/53 should be used.
 
 config ARCH_MX50
 	bool