thinkpad_acpi: Use struct dev_pm_ops instead of legacy PM routines

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

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Acked-by: Matthew Garrett <mjg@redhat.com>