ARM: imx3x: clean up ARCH_MX3X

Move to SOC_SOC_IMX3X.
Leave ARCH_MX31/35 definitions there, in case some place prevent multi-soc
single image.

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>