soc: nxp_s32: rename s32k to s32k3 for series alignment

To accommodate support for S32K1 devices, it is necessary to rename
the existing `s32k` directory, which currently houses support for
the S32K3 series, to align with the respective series names. This
adjustment is necessary given the distinct differences in core
architecture, MPU, peripherals, and other key aspects between
the two series.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>