ARM: imx: call mxc_device_init() in soc specific function

mxc_device_init() is a core_initcall function used to register devices
for mxc_aips_bus and mxc_ahb_bus, which are needed by gpio and dma
device registration.

Instead of being a core_initcall function, we have it called in soc
specific initialization function before gpio and dma devices get
registered, so that it will not be called for other platforms when
we enable multi-platform support for imx.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>