soc: arm64: nxp_layerscape: remove redundant soc.c source

The soc.c source file did nothing, so just remove it. Remove soc
directory from the include list as well, since there's no header in the
directory that needs to be exposed to the Zephyr build.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>