soc: arm: nuvoton_npcx: remove SOC_POWER_MANAGEMENT

Remove a redundant symbol that was used as a proxy to enable CONFIG_PM
and CONFIG_PM_DEVICE. If an application needs to enable PM, it should
just enable PM subsystem Kconfig options. Furthermore, there's no clue
"SOC_POWER_MANAGEMENT" is a NPCX specific option.

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