firmware: scpi: add CONFIG_OF dependency

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.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 8bec4337 ("firmware: scpi: add device power domain support using genpd")
Acked-by: Sudeep Holla <sudeep.holla@arm.com>