soc: imx: check ls1021a

fsl,ls1021a is a mach under arch/arm/mach-imx/, however it could
not use the soc driver which will break caam on ls1021a platform.

So directly return if it is compatible with fsl,ls1021a.

Fixes: 52102a3b ("soc: imx: move cpu code to drivers/soc/imx")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Horia Geantă <horia.geanta@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>