olpc-xo15-sci: Use struct dev_pm_ops for power management

Make the OLPC XO15 SCI driver define its resume callback through
a struct dev_pm_ops object rather than by using a legacy PM hook
in struct acpi_device_ops.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>