tpm_atmel: Use struct dev_pm_ops for power management

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

This allows the driver to use tpm_pm_suspend() and tpm_pm_resume()
as its PM callbacks directly, without defining its own PM callback
routines.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>