arm/imx: fix the references to ARCH_MX3

The config symbol ARCH_MX3 has been removed by commit 'a89cf59
arm/imx: merge i.MX3 and i.MX6', and it should not be referenced
any more.

The patch also change ARCH_MX* to SOC_IMX* for other platforms.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>