arm/imx: merge mm-imx35.c into mm-imx31.c

As imx31 and imx35 have much in common at soc level, this patch merges
mm-imx35.c into mm-imx31.c, so that the common functions between imx31
and imx35 can be added in one file later.

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