armv8: fsl-layerscape: Organize SoC overview at common location

SoC overviews are getting repeated across board folders.
So, Organize SoC overview at common location i.e. fsl-layerscape/doc

Also move README.lsch2 and README.lsch3 in same folder.

Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>