soc: raspberrypi-power: add CONFIG_OF dependency

We get a harmless warning if the RASPBERRYPI_POWER driver is enabled without
CONFIG_OF during compile testing:

warning: RASPBERRYPI_POWER 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>
Acked-by: Eric Anholt <eric@anholt.net>