arm/imx: remove cpu_is_xxx() check from __imx_ioremap()

This patch adds an ioremap hook imx_ioremap to be called in
__imx_ioremap().  Any soc that needs a customized ioremap other
than __arm_ioremap() can set up this hook in soc specific call.

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