memory: emif: Use kernel_can_power_off()

Replace legacy pm_power_off with kernel_can_power_off() helper that
is aware about chained power-off handlers.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>