gpio/mxc: get rid of the uses of cpu_is_mx()

The patch removes all the uses of cpu_is_mx().  Instead, it utilizes
platform_device_id to distinguish the different gpio types, IMX1_GPIO
on i.mx1, IMX21_GPIO on i.mx21 and i.mx27, IMX31_GPIO on all other
i.mx SoCs.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>