soc: nxp_s32: consolidate part number options

Currently, the NXP S32 SoCs have three redundant Kconfig hidden
options to define the part number. To streamline this, we will
retain `CONFIG_SOC_PART_NUMBER` to store the part number as a
string and `CONFIG_SOC_PART_NUMBER_<part>` that can be selected
by the boards.

Furthermore, for drivers requiring conditional code compilation
based on the target SoC, they should utilize the series or SoC
config option as applicable, instead of the part number config.

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