sh-sci / PM: Use power.irq_safe

Since sci_port_enable() and sci_port_disable() may be run with
interrupts off and they execute pm_runtime_get_sync() and
pm_runtime_put_sync(), respectively, the SCI device's
power.irq_safe flag has to be set to indicate that it is safe
to execute runtime PM callbacks for this device with interrupts off.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Magnus Damm <damm@opensource.se>