gpio/mxs: get rid of the use of cpu_is_xxx

It removes the use of cpu_is_xxx from gpio-mxs driver and instead use
platform_device_id to identify the device.  Accordingly, mxs platform
code is changed to register gpio device with different names, and
the registeration are done in soc specific initialization functions
now, so postcore_initcall(mxs_add_mxs_gpio) gets removed.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>