soc: arm: microchip_mec: mec1501: remove SOC_POWER_MANAGEMENT

Remove a redundant symbol that was used as a proxy to enable CONFIG_PM.
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 Microchip specific option.

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