imx: reorganize IMX code as other SOCs

Change is consistent with other SOCs and it is in preparation
for adding SOMs. SOC's related files are moved from cpu/ to
mach-imx/<SOC>.

This change is also coherent with the structure in kernel.

Signed-off-by: Stefano Babic <sbabic@denx.de>

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Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>