We get a harmless warning if the ARM_SCPI_POWER_DOMAIN driver is enabled
without CONFIG_OF during compile testing:
warning: (ARM_SCPI_POWER_DOMAIN) selects PM_GENERIC_DOMAINS_OF which has unmet direct dependencies (PM_GENERIC_DOMAINS && OF)
There is no need to select PM_GENERIC_DOMAINS_OF if OF is set, so we can
replace the 'select' with a dependency.