sonypi: Use struct dev_pm_ops for power management

Make the sonypi driver define its PM callbacks through
a struct dev_pm_ops object rather than by using legacy PM hooks
in struct acpi_device_ops.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Mattia Dongili <malattia@linux.it>