soc: imx: Remove Layerscape check

Since commit 4ebd29f9 ("soc: imx: Register SoC device only on i.MX
boards") the soc-imx driver is only registered on i.MX platforms as
intended.

This means that we no longer need to do a specific check for
Layerscape.

Remove the now unneeded "fsl,ls1021a" check.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>