ARM: imx: use SOC_IMX25 instead of ARCH_MX25 for multi-SoC

Some usages of ARCH_MX25 are assuming that if it is
defined the other SoCs are undefined.  Use SOC_IMX25 for the save
places.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>