i2c-ocores: Use struct dev_pm_ops for power management

Make the OpenCores I2C controller 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>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>