soc: arm: nuvoton_npcx: remove Kconfig help re-definitions in defconfig

The SoC Kconfig.defconfig.series files re-defined multiple Kconfig help
entries. The help for such symbols should only be present in places
where symbol is actually defined, not where the defaults are changed.

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